Appeared in Summer 2003, Vol. XXVIII, No. 2

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Sylvester Maurus was among the most renowned of the great philosophers and theologians who in the seventeenth century graced the Roman College of the Society of Jesus, now known as the Gregorianum. He was born in Spoleto, Italy, in 1619 to a family of the nobility. As a boy he attended schools of grammar and rhetoric, but at the age of sixteen he placed his name for reception into the Society of Jesus. After spending the usual period in the Jesuit novitiate, he studied Greek and Latin and was sent on to the order’s Roman College, where he studied philosophy and theology until 1648. There then followed in his life a period of preparation in which he took on pastoral duties for the first and only time. The Society had marked him out for the intellectual life, and so in 1652 he returned to the Roman College, this time to direct the studies of the younger scholars.

His term of apprenticeship in the intellectual life soon came to an end. Only a year after returning to the Roman College he was elected to the body of Masters, and for the next five or six years expounded the whole course of Aristotelian philosophy. He took final vows in the Order in 1654, and five years later was promoted to the Chair of Theology, which he retained for the next twenty-five years, almost to the end of his life. Two years before his death he was named Rector of the College and oversaw the remodeling of the chapel of St. Ignatius in the Jesu. He is said to have died with great piety on January 13, 1687. Fr. Maurus’ theological and philosophical insights were preserved by the publication of several volumes of his writings. Toward the end of his life he edited a collection of his theological works, but among the few who now know about him he is best remembered for the writings which sprang from his years of teaching Aristotelian philosophy. The text that I have translated is contained in his Complete Paraphrase of the Works of Aristotle, first published during his lifetime, but republished in 1885 in the series Library of Scholastic Theology and Philosophy.1 We should note that Fr. Maurus’ commentary is a mean between the close textual analysis of St. Thomas’ Aristotelian commentaries and the exhaustive paraphrases of St. Albert’s. Fr. Maurus divides and orders the Aristotelian text as St. Thomas does, but he also offers a paraphrase in the style of St. Albert. The differences are that his divisions are not as detailed as those of St. Thomas and his paraphrases are more concise than those of St. Albert. The value of Fr. Maurus’ paraphrase comes from a style which illuminates the obscurities of Aristotle’s text even for the beginner in philosophy.

The text translated below is a commentary on the Isogoge of Porphyry, a neo-Platonist of the third century A.D. commentary on the Isogoge of Porphyry, a neo-Platonist of the third century A.D. Porphyry wrote it in order to introduce beginners to the logical doctrines of Aristotle, especially the doctrines contained in his Categories. The Isogoge gave rise to the great medieval controversy on universals. It is included in most Greek and Latin editions of Aristotle’s logical works.

Fr. Maurus’ commentary, like all good scholastic commentaries, begins with a prologue in which he tells us who the author of the book is, what the book is about, to which part of philosophy it belongs, and how the book is divided and ordered. Since the book is an introduction to Aristotle’s logic, he also takes time to describe the structure of logic and explain the place occupied in that structure by each book in the collection of Aristotle’s logical treatises, called the Organon. Fr. Maurus in this way gives us a sketch of the whole of Aristotelian logic and the place of the Isogoge in it.

In the nineteenth-century edition of the works of Fr. Maurus the editors explain that, since Aristotelian logic is the logic of philosophy, it is profitable to republish his exposition of that difficult subject. In the last one hundred years, however, so much has changed in the world of philosophy that this reason by itself is no longer compelling. Bertrand Russell and Alfred North Whitehead’s Principia Mathematica has revolutionized logic and made many of the issues with which Fr. Maurus is concerned seem obsolete. And so we need to consider why it might still be important to study Aristotelian logic.

This introductory essay aims at giving us good reasons for such an assessment. It will argue that the motives for abandoning the old logic are deficient, and that the principles justifying its abandonment can actually be used to defend the old logic. Furthermore, it will turn out that the collateral benefits of the old logic outweigh those of the new. Before we make that defense, however, we need to look at why modern logicians no longer take Aristotle as their master.

The Apparent Obsolescence of Aristotelian Logic

From the point of view of the historian of logic, Fr. Maurus’ project – teaching the logic of Aristotle – would be absurd in our time. A return to Aristotle’s logic could only be obscurantism, a denial of the obvious progress made in the discipline. For the historians of logic, modern developments are not contradictions of what is sound in Aristotle, but instead are corrections of his imperfections, corrections of which Aristotle himself would have approved. To them, reaching into the past to study the older logical doctrine is not an inquiry into logic, but an inquiry into the history of logic.

Most modern practitioners of logic, however, would disagree with the assessment of the historians of logic. The founders of modern logic believe, I think rightly, that the new logic departs in a fundamental way from the Aristotelian tradition, contradicting and not developing the old doctrines. For example, Russell himself writes:

The trivial nonsense embodied in [the classical] tradition is still set in examinations, and defended by eminent authorities as an excellent “propaedeutic,”i.e., a training in those habits of solemn humbug which are so great a help in later life…Ever since the beginning of the seventeenth century, all vigorous minds that have concerned themselves with inference have abandoned the mediaeval tradition…2

Russell thinks that the logical tradition is discontinuous, that in the seventeenth century, during the very lifetime of Fr. Maurus, the most vigorous philosophical minds had abandoned traditional Aristotelian logic in favor of a logic that is fundamentally new and entirely superior. The students of the new logic would say that the reclamation of the older tradition is wrongheaded precisely because it is a return to “habits of solemn humbug.”

Leaving Russell’s rhetoric aside, we should ask what precisely the modern logicians think makes the newer logic superior to the Aristotelian tradition. Russell himself describes the problem with Aristotelian logic being its “incredible accumulations of metaphysical lumber.”3 If we take “metaphysical lumber” to refer to extraneous philosophical principles, he seems to be making two claims. First, he is claiming that Aristotelian logic is burdened with philosophical prejudices. Second, he is claiming that the new logic, which is derived from mathematics instead of metaphysics, is an independently rigorous science, posited prior to every philosophical doctrine and free from metaphysical bias. It is precisely its independence from philosophy that makes the new logic superior to the old.

Let us examine in more detail Russell’s argument for the superiority of modern logic. He contends that the purpose of philosophy is to give us an “account of the world of science and daily life.”4 Logic is the method which we use to construct and, in some ways, to verify that account. Since logic is supposed to be the standard by which we measure the validity of our philosophical inferences, logic itself should never be constructed for the sake of confirming the philosophical presuppositions of the logician. Russell holds as a principle that logic should be philosophically neutral.

He then contends that the Aristotelian tradition in logic is not philosophically neutral. He writes:

In [Aristotelian] logic, hypotheses which seem prima facie possible are professedly proved impossible, and it is decreed in advance that philosophy must have a certain character. In modern logic, on the contrary, while the prima facie hypotheses as a rule remain admissible, others, which only logic would have suggested, are added to our stock…5

That is, using the older logic, philosophers dismissed certain philosophical theories as impossible, not because unbiased inquiry into their subject matter showed them to be so, but because the rules of inference themselves could not tolerate such hypotheses. In other words, the Aristotelian logic was used to vindicate the very metaphysical doctrines upon which it was built. Therefore, the Aristotelian logic could not be an unbiased method of philosophical inquiry. On the other hand,  according to Russell, modern logic is philosophically unbiased. It was developed by mathematicians apart from philosophical theories, it elaborates logical structures for philosophical hypotheses not yet even conceived, and therefore it never rules out any hypothesis which is not immediately self-contradictory. Only modern logic allows an unbiased inquiry into the truth of philosophical hypotheses. Thus, he can conclude, “The new logic provides a method which enables us to obtain results that do not merely embody personal idiosyncrasies, but must command the assent of all who are competent to form an opinion.”6

As evidence of the bias in Aristotelian logic, Russell points out that it makes sense only if we assume two philosophical doctrines: the existence of “common qualities,” essences common to different individuals, and the capacity of the human mind to know those essences through a process called “abstraction.”7 It is impossible to make an unbiased inquiry into these subjects using the old logic because its very rules of inference force us to affirm these doctrines at the outset of the inquiry. Logicians in the classical tradition built essences and abstraction into their logic, and philosophers in that tradition then used that logic to argue that there must be essences and abstraction. The use of Aristotelian logic produces a vicious circle in philosophical reasoning.

Russell gives us an example of the philosophical neutrality of modern logic on these very same topics when he explains the principle of abstraction, or as he renames it, “the principle which dispenses with abstraction.” He writes:

When a group of objects have that kind of similarity which we are inclined to attribute to possession of a common quality, the principle in question shows that membership of the group will serve all the purposes of the supposed common quality, and that therefore, unless some common quality is actually known, the group or class of similar objects may be used to replace the common quality, which need not be assumed to exist.8

As we noted before, Russell believes that the existence of essences and our ability to know them by abstraction were the doctrines at the basis of the Aristotelian tradition in logic. Thus, any philosopher who used Aristotelian logic was forced to admit these doctrines, not indeed in virtue of an unbiased inquiry, but because of the bias in his logic. He points out that, on the contrary, modern logic makes no stipulation that essences exist or that the mind abstracts. It uses the philosophically neutral concept of “membership of the group” to account for logical inferences. The result is that a philosopher using the new logic can delve into the questions of essence and abstraction without the rules of inference themselves predetermining his answer.

We can now sum up the objection of the modern logicians. Because the Aristotelian tradition in logic has certain philosophical doctrines built into its very rules of inference, a philosophical inquiry which uses that logic as its method cannot help concluding to the truth of those doctrines. But, of course, such a procedure is a vicious circle. Modern logic, on the other hand, because it was developed by mathematicians, does not assume any philosophical doctrines. It can be used as an unbiased method of inquiry into every philosophical hypothesis, and so it is superior to the old logic.

The conclusion of this argument has the implicit approbation of the vast majority of contemporary logicians. Moreover, the Aristotelian logicians themselves probably would have recognized its strength. In fact, St. Thomas taught that since the extension of knowledge can only come about through the use of a method of inquiry, logic must come before philosophy in the order of study.9 Aristotelians would agree that a philosophically biased logic is a bad logic.

Thus, our task in the next section of this essay is to discover whether Aristotelian logic is truly philosophically biased and modern logic philosophically neutral. We will refute that claim by showing that the Aristotelians are not influenced in their study of logic by any prior philosophical doctrines, while modern logicians distort logic for the sake of maintaining certain philosophical prejudices. In order to reach these conclusions, however, we must first define what it means for a logic to be philosophically unbiased.

The Meaning of Philosophical Neutrality in Logic

We can better understand the philosophical neutrality of logic if we first understand its beginnings. A convenient way to approach the beginnings of logic is by examining St. Albert’s response to a paradox dealing with the possibility of logic.

Like every other science, logic must be acquired by the logical method. That is, the inferences that give rise to conclusions in the science of logic must themselves follow the rules of logic. But the inferences cannot follow those rules unless logic itself is already known.Thus, we can only study logic profitably if we already know it, in which case it would be pointless to do so. The attempt to learn logic seems to lead to another vicious circle.

Bertrand Russell himself takes the first step in answering this objection. He writes:

In every philosophical problem, our investigation starts from what may be called “data,” by which I mean matters of common knowledge, vague, complex, inexact as common knowledge always is, but yet somehow commanding our assent as on the whole and in some interpretation pretty certainly true.10

Russell makes a profound point here. In every philosophical investigation we begin with some given, common knowledge, which is certain, but vague and imprecise. The task of philosophy is to extend our knowledge by perfecting the knowledge that we already have, making it distinct and precise.

St. Albert applies this principle to logic. He makes a distinction between the acquired art of logic and what he calls “natural logic,” writing:

Avicenna says that this method [logic] is inborn in all men, because our understandings are in some way by nature. But what exists in nature is imperfect; it is perfected through an acquired art…. And thus this method of logic begins in nature, but is perfected by art, and also receives perfection through use and exercise.11

St. Albert is telling us that there are two logics, the acquired art of logic, of which we have been speaking, and a natural logic. The latter is almost innate and is nearly as certain as our first knowledge of the world. Like all common knowledge, however, natural logic by itself is incomplete, being vague and imprecise. The acquired art of logic perfects this natural ability, making its rules of reasoning distinct and precise.

St. Albert then uses this distinction to resolve the initial paradox. The art of logic cannot at first judge the discourse by which we come to acquire that art, because that art has not yet been acquired. But natural logic, being almost innate, can guide us in making the good inferences that lead to the acquired art of logic. As the acquisition of the art and the practice of it bring our natural ability for good reasoning more and more to perfection, the rules of logic begin to reflect upon their own derivation. By the time we complete our study of logic, we possess the acquired art and can justify that art by its own rules.

In the prologue to his Commentary on the Posterior Analytics of Aristotle, St. Thomas explains at more length the role of reflection in the acquisition of logic.12 He notes that all arts and sciences take their beginnings from reflection upon imperfect human actions. For example, by nature we are driven to build shelter for ourselves, but when we reflect on our action and its imperfect results, we sees ways to improve it. When human reason directs the action of making shelter so that it more perfectly and easily achieves its aims, the art of building has come into existence. In general, the arts are acquired by rational reflection on natural, but imperfect action.

Reasoning itself, however, is an action. Human reason, then, is able to reflect not only on other human actions but also on its own. By reflecting on how it naturally guides its own discourse and seeing how that guidance might be perfected, human reason comes to formulate the acquired art of logic.

We may suspect that there is a problem with St. Thomas’ account. Most human activities, such as the arts of building and pottery, take place outside the soul, where they can be perceived by the senses. The activity of reasoning, however, remains inside the soul and thus is not subject to the scrutiny of the senses. Since reason always reflects upon what is presented to it in sensation, it seems that the art of logic, which directs the activities of reason, cannot be acquired merely by reflection.

The solution to this problem is found in the use of words. Words are the tools which reason uses to attain a better understanding of the world around us and to pass on that understanding to others. Words are the necessary expressions of the interior activity of reasoning. Thus, logic does not come to be by a direct reflection on the activities of reason, but by reflection on their outward expressions, the words in which our thoughts are clothed. Logic is a reflection on the uses of words.

We can now see what it means for a logic to be philosophically unbiased. Our understanding of how words work is the common knowledge upon which logic is based. Logic begins in a reflection upon the natural way in which human reason uses words to come to understand the world better. Therefore, logic must give an account that includes every such use of those words, and it must not dismiss any use as meaningless. That is, it must make our common knowledge of natural logic distinct and precise and must not contradict any large part of that common knowledge. Such a logic would be truly philosophically unbiased.

In contrast, a logic which took for granted a philosophical doctrine and because of that doctrine dismissed as meaningless the common ways in which words guide our understanding would be a logic that is philosophically biased. A logician in the tradition of Aristotle could then agree with Russell that a philosophically biased logic would be, at best, radically incomplete and probably absolutely false. This is the real difference between logics that are philosophically biased and unbiased.

Let me sum up our discussion so far. Logic, being the method of acquiring philosophical knowledge, must be prior to any philosophical theory. Moreover, the study of logic, like every philosophical investigation, takes for granted some kind of common knowledge and tries to extend and perfect that knowledge. The common knowledge from which logic begins is embodied in the ordinary way in which we use words to guide ourselves to a better understanding of reality, and this ordinary use of words is a kind of natural logic. An acquired art of logic which is philosophically unbiased will give an account which does not dismiss any large feature of natural logic. A logic which dismisses as meaningless major features of our natural method of discourse because of a prior philosophical theory will, on the contrary, be a logic that is philosophically biased and radically inferior to the first.

The Neutrality of Aristotelian Logic

My next task is to construct a defense of Aristotelian logic. My defense happens also to be an attack on the idea of replacing Aristotle’s logic with the modern one. My argument will reverse Russell’s: he argued that the older logic was biased, while the modern logic is unbiased, but I will argue that the modern logic is philosophically biased, while Aristotelian logic is unbiased. While my argument will concern only one part of logic, that part is something that both traditions agree is fundamental. It will contrast how Aristotelian and modern logic deal with the universal word.

Let us begin with a very ordinary universal word, ‘dog.’ All of the following statements are true: Fido is a dog, Spot is a dog, and Rover is a dog. Note that the Aristotelian tradition uses the phrase “predicated of ” in a precise way. Only when the subject is connected to the attribute by means of a ‘to be’ verb will the attribute be “predicated of ” the subject in the strict sense of that phrase.13 Thus, in the terminology of Aristotelian logic, ‘dog’ is one word predicated of many things, namely, Fido, Spot, and Rover, while Fido, Spot, and Rover are predicated of nothing but themselves. But to be apt to be predicated of many is to be universal, while to be apt to be predicated of one only is to be individual. Thus, while ‘Fido,’ ‘Spot,’ and ‘Rover’ are individual words, ‘dog’ is a universal word. The distinction between the universal word and the individual word is the first distinction which Aristotelian logic makes.

The logicians of the Aristotelian tradition explain that the use of the word ‘is,’ except in the cases in which it only signifies the existence of a thing, points to an identity between the subject and the predicate. For example, the statement “Fido is a dog” posits an identity between Fido and dog. This understanding of the universal word, however, immediately leads to a logical problem. For if Fido is identified with dog and Spot is also identified with dog, it seems that we must identify Fido with Spot. In other words, if Fido is a dog, and Spot is a dog, then Fido is Spot. That inference is invalid, of course, but it seems to be the only one consistent with this account of predication. We bring up this problem because seeing how Aristotelian logicians solve it will help us to understand the
philosophical neutrality of their tradition.

St. Thomas Aquinas solves the problem by making a distinction between signifying something determinately and signifying it indeterminately. The word ‘dog’ points to the same whole which the word ‘Fido’ points to, but it points it out, signifies it, in a different way. ‘Fido’ points determinately to everything that belongs to Fido, explicitly including even those things which make Fido distinctive from Spot, while ‘dog’ points determinately only to some of what belongs to Fido, and signifies what is distinctive indeterminately, neither including nor explicitly excluding it. For example, ‘Fido’ points determinately to Fido’s particular color, brown, while ‘dog’ points to the color indeterminately, neither including nor explicitly excluding ‘brown.’ A similar explanation would hold true for ‘dog’ being predicated of ‘Spot.’ That is why, logically speaking, ‘dog’ can be predicated of both Fido and Spot.

We should notice that the solution which Aristotelian logic gives to this problem simply refers to how words are used. Words can point to things either in an explicit and determinate way, or with an openness to further specification. But the solution to the logical problem of the universal word gives rise to the metaphysical problem of the universal, the problem of explaining what in reality makes the use of universal words possible. In his Isogoge Porphyry gives us the classic presentation of the metaphysical problem of the universal, dividing it into three questions: whether universals exist in reality or are only concepts in the mind; if they are real, whether they are corporeal or incorporeal; and if incorporeal, whether they are separate from bodies or existing in and around them.

But even though he presents the metaphysical problem in the Isogoge, Porphyry’s reaction is to postpone giving any solution to it. “That business,” he writes, “is very deep and requires a greater examination.”14 Porphyry is implying that, since this problem of the universal is not a question about how words are used, it is not a logical question, but rather a metaphysical one about the nature of real things and how the human mind relates to them. Although the Aristotelian philosophers later conclude that the universal word can only find its ultimate explanation in a doctrine of essences and abstraction, they do not use that doctrine either explicitly or implicitly to solve the logical problem of the universal. The philosophical doctrine is neither a part of Aristotelian logic nor a presupposition of it.

Thus, Porphyry’s treatment of the problem of the universal is a perfect illustration of the philosophical neutrality of Aristotelian logic. It looks at the phenomena of words as they are given in everyday life. We use universal words and we classify things with them. Their correct use, therefore, must correspond to logical rules. It is the business of a philosophically unbiased logic to formulate those rules, not to explain the features of the reality and the human mind which make such rules necessary. The latter is a philosophical problem, and logic is prior to philosophy. Yet neither is it the business of logic to dismiss these features of common language as meaningless because of a prior commitment to a philosophical theory. Such a logic would be no longer philosophically unbiased.

Porphyry’s insistence on the priority of logic is the characteristic method of Aristotelian logic. It takes the phenomena of words and uses them to determine the subject matter of logic. A particular account in the Aristotelian logical tradition is judged to be good if it can explain the given uses of words. It is judged deficient if it omits or dismisses as meaningless any common way of using words to guide reasoning. Aristotelian logic carefully excludes philosophical bias.

The Bias of Modern Logic

We will discover, however, that the situation is much different when we look at modern logic. The modern logician comes to his subject with philosophical prejudices that determine how he constructs his logic. His logic is forced to dismiss as logically meaningless certain common ways of using language to guide our thinking and thus it becomes, not the completion of our natural method of reasoning, but an impoverished replacement for it.

An example of the bias of modern logic is the way in which it deals with the problem of the universal. Modern philosophers usually address the metaphysical problem of the universal before the logical problem, and then present a solution to the logical problem which presupposes their metaphysical doctrines. Ultimately their solution forces them to assert that the everyday use of the universal word is logically meaningless.

Bertrand Russell unwittingly illustrates this process in his discussion of the principle of abstraction.15 He rightly judges that the question of whether many real things can share a common quality, an essence, is a metaphysical question. He does not realize, however, that when he claims that “the membership of a group” can replace essence, he himself is making a metaphysical claim. Thus, when he constructs his logic of predication explicitly upon the notion of membership of a group, he has actually built a metaphysical doctrine into his logic. It is Russell’s logic that truly is burdened by “accumulations of metaphysical lumber.” And, as we are about to see, these prejudices lead to the denial of the meaningfulness of a common feature of ordinary language, the universal word.

To make this point clear, let us again examine the universal word ‘dog.’ Russell must account for the universality of ‘dog’ by using the concept of the membership of a group. Fido, Spot, and Rover are all called dogs because they are gathered together in a mental group which for convenience we associate with the word ‘dog.’ What is important for Russell is that there is nothing about groups as such that requires that their members have the same essence or share a common quality. Any variety of things, and even non-things, can be thrown into a group. For example, one could make a collection of Fido the dog, Felix the cat, a unicorn, the tree in my yard, the blueness of my car, and the idea of justice, and then one Aristotle could assign a symbol, X, to that group. Just as we say that Fido belongs to group ‘dog,’ so also the idea of justice belongs to group X. And just as the members of group X have no common quality, so also there is nothing about the collection symbolized by the word ‘dog’ that requires that Fido, Spot, and Rover have a common quality.

Modern logic runs into a problem, however, when it encounters the idea of predication, a concept which falls into the very definition of the universal word. A group relates to its members as the quantitative whole relates to its parts. That is, Fido, Spot, and Rover are related to the group named ‘dog’ in the same way that arms, legs, a torso, and a head are related to Socrates. The parts put together make up the whole. But that kind of whole is never predicated of its parts, while a universal word is always predicated of individuals. For example, just as we do not say “Socrates is his arm” or “Socrates is his legs,” so we do not say “Fido is the group that contains Fido, Spot, and Rover.” But we do say “Fido is a dog.” Consequently, ‘dog’ cannot be the group containing Fido, Spot, and Rover. Thus, the account which the new logic gives of the universal word actually forces it to ignore the phenomenon of predication, the defining feature of that kind of word. The new logic does not explain the universal term, it explains it away. Modern logic must say that the universal word is logically meaningless.

Let us recall why Russell first posited the principle of abstraction. He did so because he had a prior philosophical conviction that common qualities could be replaced by groups. Only a logic which reduces the universal to a group makes it possible to avoid the implication that essences and abstraction are real. Thus, because Russell’s logic makes the universal equivalent to the group, he ends by explaining away the fact that in ordinary speech we predicate the universal term of the individual subject.

The Superiority of Aristotelian Logic

We are now ready to make our argument for the superiority of Aristotelian logic. Philosophers rightly demand that logical doctrine be prior to philosophical doctrine and be philosophically unbiased. But a logic is only philosophically unbiased if it takes into account all the ways in which we use words to better understand things. That logic is philosophically biased which, because of a prior commitment to a philosophical theory, must in principle ignore certain common features of human discourse and inquiry. Aristotelian logic does not in principle eliminate any feature of human discourse, but tries to explain them all. The modern philosophers, such as Bertrand Russell, designed modern logic to fit their prior philosophical convictions. Thus modern logic excludes from its consideration those features of human discourse which imply anything contrary to the logician’s philosophical prejudices. If we use philosophical neutrality as our criterion, it turns out thatAristotelian logic is superior to modern logic. Thus it turns out that, as in so many other cases of things modern, the claim of the new logic to neutrality simply masks a new kind of prejudice.

So far the defense of Aristotelian logic has been successful. But we have yet to answer directly the charges of bias leveled against that logic, to show that the accusations against it are false. To give such an answer is the main task of the next and last section of this introductory essay, but that answer will also lead us to conclude that what at first seemed to be a liability for Aristotelian logic turns out to be an advantage.

The Answer to Russell’s Objection

Russell pointed to the universal word as an example of the metaphysical bias of the older logic because the Aristotelian explanation of the universal word must fall back on the metaphysical doctrine of essences. But if the use of the universal word implies the existence of essences, then a logic that includes the universal word is biased toward essentialist metaphysics, and if that logic is used to argue for such a doctrine, it must beg the question.

We have already referred to the principle behind our answer. The old logic does not presuppose any philosophical theory about essences and therefore is not metaphysically biased. Its only presupposition is that it must explain the ways in which men naturally speak about things. If its teaching about the universal word can ultimately be explained only by reference to essences, this is because our natural logic, our everyday ways of using words to understand things, cannot be fully accounted for without reference to essences. The fault, if fault there be, is not in the old logic, but in the way in which we naturally speak.

In fact our solution to Russell’s objection points to an advantage that the old logic has over the new. The new logic was designed with a certain philosophical view in mind, a kind of empiricism. Empiricism declares that our knowledge is restricted to sense perception of the individual, and that the common essence either is unreal or unknowable. Because the new logic was designed with its matching philosophy in mind, philosophers cannot draw new philosophical insights from reflecting on it. The new logic contains only what the empiricist philosophy put into it.

The old logic is much different. It takes no philosophy as its starting point, but instead begins from our everyday ways of speaking. When the philosopher reflects upon the doctrines of the old logic, he is reflecting upon something whose contents were not predetermined to fit a philosophical theory. Consequently, the doctrines of Aristotelian logic are able to give the philosopher new insights or to contradict his theories. The philosopher can use the old logic, not just as a method for reasoning clearly in metaphysics, but as material from which he can derive new metaphysical insights.

Both Aristotle and St. Thomas explicitly make such a use of Aristotelian logic. Aristotle, in the seventh book of the Metaphysics, when he begins to determine about being as such, first approaches that subject by trying to gather insights from the doctrines of logic. In his Commentary on the Metaphysics St. Thomas points out that:

Aristotle, in the seventh book, determines about substance in the logical mode, namely by considering definition and the parts of a definition, and other such things which are considered from the viewpoint of reason.16

Aristotle wants to figure out the essence of substance, but he does not begin by looking at it from the point of view of its proper principles. Such a course is too difficult for the metaphysical beginner. Rather, he first examines the doctrines of the science of logic on subjects such as predication and definition. These give him insight into the natures of the essences which they signify.

St. Thomas himself follows this same method in his On Being and Essence. When he outlines his treatise, he tells us that the second chapter will deal with the essences of sensible substances, while the third will discuss logical matters. But in fact every reader quickly notices that his discussion of essence in the second chapter is also filled with references to the doctrines of logic. St. Thomas, like Aristotle, looks to the doctrines of logic, especially the doctrines of definition and predication, to give him insight into the nature of essence in reality. Both philosophers testify that the old logic can be the source of new philosophical insights.

We can conclude our defense of Aristotelian logic by admitting that the logical phenomena which it partially accounts for cannot be fully explained except by some doctrine of common essence. But Russell is wrong to attribute this to the bias of Aristotle; the fault rather lies at the feet of our ordinary use of language, especially of the universal word. We might sum up our defense by saying that, while the connection between empiricism and modern logic is a source of weakness, the connection between Aristotelian metaphysics and Aristotelian logic is a source of strength.

Our Return to Fr. Maurus

If I am right about the relative merits of the old and new logics, it should be clear that it is worthwhile to read Fr. Maurus’ commentary on the Isogoge of Porphyry. The Aristotelian logical tradition deserves serious study both for the soundness of its own doctrine and as a source of philosophical insight. That doctrine has been almost obliterated because of the philosophical prejudices of our time. Fr. Maurus is one of the last masters of the old logical tradition, a master of that tradition precisely when it was being abandoned by so many. He is one of our few remaining links to that tradition. It makes sense for us to renew our links with that tradition precisely where Fr. Maurus begins, with a discussion of the universal in the context of the Isogoge of Porphyry.

On the Logic of Aristotle and the Isogoge of Porphyry

By Sylvester Maurus, S.J.


Note: This prologue proposes a division of logic and of the books of Aristotle; it explains the subject of Porphyry’s Isogoge; and it examines whether the Isogoge is part of logic.

1. The learned disciplines should be ordered and divided just as the things that are the objects of these disciplines are divided. Now logic, the discipline concerned with reasoning, has for its object either the actions of the rational power, that is, the operations of the intellect; or, according to another opinion, the beings of reason called second intentions, things and objects insofar as they are named by the actions of reason and of the intellect. In either case logic should be ordered and divided in the same way as the actions of reason, since these actions are either the objects of logic, or they at least make the beings of reason (second intentions) which are the objects of logic. But the text of Aristotle divides the actions of reason or the operations of the intellect into three: first, apprehension, or simple understanding; second, composition and division, or immediate judgment; and third, discourse, or mediate judgment.17 Therefore logic should also be divided into three parts. In the first part things are discussed insofar as they are simply understood; in the second part things are discussed as enunciated by the second operation; in the third part things are discussed as syllogized about and concluded to by means of the third operation.

2. Following this order Aristotle gives us the first part of logic in his book, Categories. There he discusses things insofar as they are simply understood or apprehended. He gives us the second part in the book called Peri Hermeneias, or On Interpretation, in which he discusses the statement and things insofar as they are stated. He gives us the third part in the Prior and Posteriorm Analytics, in the Topics,and in the Refutations, in which he discusses the syllogism in general and the various species of it, namely, the demonstrative syllogism, the topical or probable syllogism, and the sophistical syllogism. Here he discusses things insofar as they are syllogizable, either demonstratively, or with probability, or only apparently syllogizable.

3. Porphyry, a philosopher who was partly Platonic and partly Peripatetic, adds to these books of Aristotle the Isogoge [Introduction] about the five predicables. The subject of this book, as is evident from its prologue, is the five predicables: genus, species, difference, property, and accident. The teaching of the book is almost entirely taken from the books of Aristotle, especially the Topics, although he sometimes falls into opinions which are Platonic and contrary to Aristotle’s.

4. Some say that this book is not a part of logic but only an introduction to logic. Still, we should say that, even though it is an introduction to the logic of Aristotle, it itself is also a book of logic. Proof, a book which discusses the object of logic is a book of logic. In this book, however, the object of logic is discussed. Therefore, this book is a book of logic. We can prove the minor premise of this argument as follows: the predicables are objects of logic, but this book discusses the predicables. We can prove the major premise [of this second argument] as follows: the objects of logic are the beings of reason, that is, things insofar as they are denominated through an act of reason of the intellect; but the predicables, genus, species, etc., are beings of reason because a thing is called a genus, a species, etc., through an act of the intellect. Therefore, etc.

5. We can also infer that this book is about the first part of logic, as Categories is. For in this book things are discussed insofar as they are simply understood, since things are called predicables, such as genus, species, etc., by reason of being simply apprehended.

6. This book is divided into seven chapters. In the first Porphyry gives us a prologue in which he explains the necessity and usefulness of this introduction and the way in which we should proceed in discussing these topics. In the second he discusses genus, in the third species, in the fourth difference, in the fifth property, in the sixth accident. In the seventh chapter he compares the five predicables among themselves and explains their likenesses and differences; that is, those things in which they are alike and those in which they differ.18

The Isogoge of Porphyry

Chapter One: Prologue

Writing to Chysaorios, Porphyry begins his Isogoge to the five predicables with a prologue in which he explains the necessity and usefulness of this introduction and the way in which he will proceed in his discussion of the subject.

1. First he teaches that the Isogoge is useful and necessary for logic. The reason is that the discipline of logic explains what is taught in Aristotle’s Categories, and also teaches about definition, division, and demonstration. In order to understand the Categories and the doctrines of definition, division, and demonstration, it is very useful, indeed necessary, to know first what genus, species, difference, property, and accident are. All these things are briefly explained in the Isogoge according to the teachings of the ancient philosophers. Thus, the Isogoge is both very useful and necessary for logic. -The minor premise of this argument is proved as follows: First, in Categories Aristotle assigns the ten highest genera and divides them through essential differences into their subalternate and lowest species. He also allots to these genera and species their proper and common accidents. Thus, if we wish to understand these things well, we must first know what genus, species, difference, proper accident and common accident are. Second, an essential definition explains the species or essence of a thing through its genus and difference, while a descriptive definition explains a species through its genus and proper passion. There are even certain descriptions which explain a thing through common accidents. Third, one kind of division is that of a genus into its species, another of a genus by its differences, another is the division of a subject through its proper and common accidents, and another is the division of accidents through their subjects. Fourth, the most powerful middle term in a demonstration is the definition of the subject, which is made from a genus and a difference. Moreover, the point of demonstration is to show that subjects possess certain proper accidents. Thus, the doctrine of the five predicables is useful, even necessary, for teaching about definition, division, and demonstration.

2. After he has explained the usefulness and necessity of the Isogoge, he explains the way in which he will approach the subject. He says that he will abstain from the higher and more difficult questions about genus and species but instead will discuss the easier questions, a method that befits the abilities of Chysaorios and others who need to be introduced to logic. Yet he does present here the more difficult questions: are genus and species real; are they bodies or incorporeal; and, are they found in sensible things, or do they exist separately from sensible things, being a middle between the sensible and the non-sensible? Answers to these and similar questions are omitted in the Isogoge, since they concern the subject of the highest science, namely, metaphysics. Even the easier questions will not be discussed very precisely, but only in the way proper to logic, just as was done by the ancients, especially the Peripatetics.

Chapter Two: About Genus

Having begun with a prologue, Porphyry now takes up his discussion of the predicables, beginning with genus. He first explains three meanings of that term, and then draws out of this a description of genus according to its philosophical meaning. After this he declares how genus is the same, and how it is different, from the other predicables.

1. Taking up the first task, he says that the terms ‘genus’ and ‘species’ are used in many ways. First, a multitude of men related to each other by common birth are called a genus, because they draw their origin from one man.19 For example, the genus Heraclidae signifies the multitude of men related to each other by common birth, taking their origin from Hercules. Through this origin they are different from other genera of men, who do not take their origin from Hercules.

2. Second, the principle of generation is called a genus, whether that principle be one’s mother and father or one’s country, which is in some way like a mother. In this sense we say that Orestes has his origin from Tantalus, and Hyllos from Hercules, since Orestes has Tantalus for his principle of generation, and Hyllos Hercules, both as parents. Yet we say that Pindar is of the Theban genus, and Plato of the Athenian, since Thebes was a principle of generation for Pindar and Athens for Plato, each being the native countries of those men. In fact the second meaning of the term ‘genus’ is more commonly used than that mentioned before. For example, those who descend from Cecropos and his relatives are called Heraclidae. It also seems that the name ‘genus’ was first imposed to signify the principle of generation, and then was carried over to signify the whole multitude of those who have flowed from the one principle of generation. For example, we first attribute the genus Heraclidae to Hercules himself, from whom the Heraclidae descend, and only then do we extend it to the whole multitude of those who descend from Hercules, in order to distinguish them from other multitudes.

3. Third, that under which a species is placed is called a genus. For example, animal is a genus, since the species man, lion, dog, etc., are placed under animal. Furthermore, that under which the species is placed seems to be called a genus because of a certain analogy and likeness to the genus that is a principle of descent and to the genus that is a multitude which descends from one man. Also, that under which the species is placed is a certain principle of the subject species. For example, animal is a certain principle for man, lion, and the other animals. And furthermore, that under which the species is placed seems to contain the whole multitude of subjected species. For example, animal seems to contain all the species of animals, man, lion, etc., and all of these animals participate in the genus as if participating in one thing.

4. Then Porphyry takes up his second task. Since the philosophers only use the term ‘genus’ in its third sense, only that sense is discussed in detail in this present work. Therefore, in its third meaning genus is described by the philosophers as that which is predicated in answer to “What is it?” of many differing in species, animal being an example. -Porphyry explains this definition and at the same time proves that it is good. For the definition of genus will be good if it explains how genus is the same as, and how it differs from, the other predicables. This definition explains that: therefore, etc. – The minor premise is proved thus: What is predicated is either predicated of one only, such as the individual (for example, Socrates, this man, that thing) or of many. If it is predicated of many, it is predicated either as a genus (for example, an individual, but rather is common to the individuals of many species (for example, white, black, sitting). But the aforesaid description explains how genus is the same as and differs from these. Therefore, etc. -The minor premise of this argument is proved thus: All of the predicables are the same in this respect, that they can be predicated of something; genus differs from the individual in this, that the individual can be predicated only of one thing, but a genus can be predicated of many. A genus differs from species in this, that a species can be predicated of many differing only in number [that is, as individuals], but a genus can be predicated of many differing also as species. For example, man can be predicated of Socrates and Plato, who differ only in number; animal, however, can be predicated of cow, man, and horse, which differ as species. A genus also differs from a property. For a property can be predicated only of the one species of which it is the property and of the individuals of that species. For example, to be able to laugh can be predicated only of the species man and of each man. A genus, however, can be predicated of many species and of the individuals of many species. A genus also differs from a difference and from a common accident. Although differences and common accidents are predicated of many differing in species, they are not predicated in answer to the question “What is it?” For example, if someone asks “What kind of animal is a man?” we respond with the difference, saying “rational.” If someone asks “What kind of animal is a raven?” we answer with an accident, saying “black.” But if the question asks “What is man?” we do not answer “He is rational”; rather, we answer “He is an animal.” Again, if someone asks, “What is a raven?” we do not answer “A raven is black” but rather “A raven is an animal.” Therefore, a genus answers the question “What is it?” while a difference and an accident only answer the question “Of what kind?” In sum, a genus differs from individuals in being predicated of many, while [it differs from species through this, that species] is predicated only of many differing in number.20 It differs from difference and accident through this, that it answers the question “What is it?” while those others do not, but rather they answer the question “Of what kind?” Therefore, the proffered definition of genus is good, since it explains neither more nor less than the concept of the genus and how it differs from the other predicables.

Chapter Three: About the Individual and the Species, Both Lowest and Subalternate

After Porphyry has discussed the genus, he then discusses the species. First, after he has explained the two meanings of species he defines the philosophical species, beginning with the predicable species and concluding with the subjectable species. Second, he explains that a highest genus and several lowest species, as well as subalternate genera and species, are found in all of the categories. He describes these and compares them to each other, showing that there is not one highest genus common to all things, but rather that there are ten, that is, there are ten categories. Third, he explains universally how genera and species, both the higher and the lower, are related to each other by way of predication. Fourth, he describes the individual and in that way concludes his discussion.

1. Let us consider the first task and note that the word ‘species’ comes from ‘spectando,’ [which means ‘looking at’]. Therefore, the word ‘species’ first means ‘visible form’ or ‘outward appearance.’ Thus, the word ‘speciosus’ first means lovely, and ‘species’ first means loveliness or beauty. Since barbarians think that the dignity of man consists more in the beauty of the body than in the virtue of the soul, they say that species is “as first, worthy to command.”

2. Second, that which is under an assigned genus is called a species by the philosophers. This is what we mean when we say that man is a species of animal, or whiteness is a species of color, or triangle is a species of figure. The reason why the nature which is under a genus is called a species is that it adds form, beauty, and the perfection of a difference to the genus, which in itself is a certain indeterminate and unformed thing. For example, man adds form, beauty, and the perfection of rationality to animal, which in itself is something indeterminate and unformed.

3. But Porphyry points out a difficulty with what he himself has just said, namely, that the genus is described by means of the species and defined as that which is predicated of many differing in species. A species, however, is described by means of its genus, since a species is that which is under a given genus. Therefore, since the same thing is explained through itself, we find a vicious circle in these descriptions. He responds, however, that correlatives cannot be defined unless each is put in the definition of the other. For example, the double is double of the half, and the half is half of the double, and so half is put in the definition of the double. Genus and species, however, are correlatives, for a genus is a genus of a species, and a species is a species of a genus. Therefore, it is not strange that they include each other mutually in their descriptions.

4. Species is also defined as that which is placed under the genus and of which the genus is predicated in answer to “What is it?” For example, man is a species of animal because it is placed under animal in such a way that animal is predicated of man in answer to the question “What is it?” So that this definition will be good and fit species alone, and not also fit the individual, “being placed under” must be understood to refer to immediate subordination. This is because the genus also answers the question “What is it?” about the individual and the individual is also placed under the genus, but only through the mediation of the species. For example, not only the species man, but also the individual Socrates, is placed under animal. Moreover, animal is predicated of both man and Socrates in answer to the question “What is it?” But Socrates is not placed immediately under the genus animal, and therefore he is not a species.

Again, species is defined as that which is predicable of many differing in number only, in answer to “What is it?” For example, man is a species because it is predicable of many men in answer to “What is it?” and those many differ only in number. Porphyry adds that this last definition only fits a lowest species, which is a species only, and not a genus. The other definition fits a subalternate species also, which is both a species and a genus at “There is not one highest genus common to all things, but rather that there are ten, that is, there are ten categories.”

5. Porphyry then moves on to the second task. He has already mentioned the lowest species, but to explain it he points out that in every category there is a highest genus, a lowest species, and subalternate species, which are species and genera at the same time, although with respect to diverse things. The highest genus is that which does not have another genus above itself; the lowest species is that which does not have another species under itself; middle and subalternate species and genera are those which bothnhave another genus above themselves and have other species below themselves. Therefore, they have the nature of a species with respect to a higher genus to which they are subjected, and they have the nature of a genus with respect to a lower species of which they are predicated.

6. He explains this doctrine through an example in the category of substance. Substance is a certain genus; under substance is body, which is a species of substance; under body is living or animate body; under living body is animal; under animal is rational animal which, according to the Platonists whose error Porphyry followed, is the common genus of two kinds of rational animal, the immortal and the mortal; under rational animal is man, that is, the rational and mortal animal; finally, under man are Socrates, Plato, and other individual men.

Among these, substance is the highest genus and is so much a genus that it is not a species; man is a lowest species, since it does not have other species under it, for men do not differ in species. Thus, man is so much a species that it is not a genus. The others are intermediate, species and genera at the same time. For body is a species of substance, but the genus of living or animate body; living body is a species of body, but the genus of animal; animal is a species of living body, but the genus of the rational animal; rational animal is a species of animal and, according to the error of the Platonists, the genus of man; according to that same error man is a species of rational animal, but not a genus. It is a species only, predicable of individual men who do not differ in species, but only in number.

Universally, what is immediately before and above the singulars is the lowest species and is not a genus. Therefore, just as substance is the supreme and highest genus, [since it does not have a genus above it, so also man is the lowest species] since it does not have a species under it.21 Some are indeed in the middle, such as animal, living, etc. They are species of those under which they are placed, and genera of those of which they are predicated. Therefore, they have two relations, one to the higher genera, according to which they are called species, and another to the lower species, according to which they are called the genera of the lower species. The extremes, that is, the highest genus and lowest species, have only one relation. Since the highest genus does not have another genus above it, it does not have the relation of a species able to be placed under a higher genus, but only the relation of the genus to the species below it. Of course, the lowest species has the relation of a species to the superior genera. Still, it does not have any other relation to its inferiors, that is, the individuals, than being a species, since just as species can be placed under a superior genus, so also species are predicable of many individuals. Therefore, it is a species containing individuals below it, but contained by the genus above it.

7. From what was said before Porphyry then deduces the definitions of a highest genus, a lowest species, and subalternate genera and species. The highest genus is that which is so much a genus that it is not a species, or that above which there is not another genus. The lowest species is that which is so much a species that it is not a genus, or that which is not divisible into other lower species; species and genera are subalternate which are genera and species at the same time, but with respect to diverse
things. For with respect to the superior genera, under which they are placed, they are species. With respect to the inferior species, of which they are predicated, they are genera. These subalternate genera and species are all of the intermediates through which the lowest species ascends to the highest genus.

This ascending can be explained by an analogy to the genealogies of families. In families we ascend from some final member up to the first beginning of the family. For example, from the final member, Agamemnon, we rise up to Zeus, the beginning of the family, by saying: Agamemnon was the son of Atreus, who was the son of Pelops, who was the son of Tantalus, who was the son of Zeus. In the same way, from the lowest species we rise up to the highest genus. For example, man is under animal, which is under living body, which is under body, which is under substance, which is the highest genus.

8. There is, nevertheless, this difference between the genealogies of families and the reductions of genera and species: many families are reduced to one first beginning of all families, say Zeus according to the errors of the pagans, or Adam according to the truth of the Faith. But species and genera are not reduced to one highest genus common to all things. For, as Aristotle teaches, there is not one highest genus common to all things and categories. Rather there are ten first genera and almost first principles, that is, the ten categories. The categories are the same in this, that they are beings; however, they are not called beings univocally, but equivocally or analogously. Therefore, being is not a genus. For if it were a genus, it must be predicated of the categories univocally and according to a definition which is entirely the same. In fact, it is not predicated according to a definition entirely the same, but rather according to definitions partly diverse; that is, it is predicated analogously.

The highest genera are ten, and the lowest species are not infinite, although they are not yet numbered. The individuals, however, are infinite: for example, the number of possible men is infinite. So Plato said that by beginning at the highest genus and dividing genera through immediate differences, we must finally descend all the way to the lowest species, and there stop. We do not descend to the individual, since this is infinite and so not knowable. For the infinite is not knowable by the finite intellect. In descending, progress is made from one genus to a greater and greater multitude of things. Conversely in ascending, in which we ascend from individuals and species to genera, progress is made from “manyness” toward “fewness,” until we arrive at what is itself one. For a species collects the infinite individuals into one, and a genus collects unnumbered species into one, etc. Conversely, many species divide one genus, and many individuals divide one species. Therefore, the lower and nearer to the singular is more and more divisive, while the higher and nearer to the highest genus is more and more unifying.

9. Having explained the division of genera into their subjected species, Porphyry now moves on to his third task, giving the rules of predication. Therefore he teaches that the genera are predicated of the species and the superior of the inferior; but a species is not predicated of its genera, neither of the proximate genus nor of its superiors. The reason is that equals are predicated of equals (for example, able to whinny is predicated of horse), and the greater or more widely extending is predicated of the lesser (for example, animal of man). But the lesser or less widely extending things are not predicated of greater things, at least not by a natural predication. For example, man is not predicated of animal. Since species extend less widely than genera, they are not predicated of their genera, although the genera are predicated of their species and of all things of which the species are predicated. For example, since man is predicated of Socrates, and animal of man, and substance of animal, therefore substance is also predicated of Socrates and man, so that it is true to say, not only that animals are substances, but also that man and Socrates are substances. Therefore, a species is only predicated of individuals, but a genus is predicated of both the species and the individuals, and the highest genus is predicated of all of the subalternate genera, of the lowest species, and of the individuals. The lowest species are only predicated of the individuals, but the individual is predicated of one thing only. Examples of individuals are Socrates, this man coming, and son of Sophroniscos, if the latter has only Socrates as a son.

10. Since Porphyry has often mentioned the individual, here he moves on to his fourth task, describing the individual. The individual is that which is made from properties, the aggregate of which is never the same in another thing. For example, Socrates has properties, the aggregate of which are never the same in another thing. We are accustomed to express the properties in this verse: The aggregate of such properties are never in another individual. For we never find two men who have the same name, and also are in the same family, have the same shape, stature, etc. Thus, the properties through which individuals of the same species differ and can be distinguished from one another are called individual, but the properties in which all the individuals of the same species agree are called specific. For example, the properties which all men have are specific.

11. From the things he has just said, it is obvious that the individual is contained under the species, the species under the genus. Therefore, the individual is part of a species, since it is contained by the species, and the genus is a whole with respect to the species, since it contains the species. Thus, the species is a whole and a part at the same time. With respect to genus it is a part, since it is contained by the genus, but with respect to the individual it is a whole, since it contains the individual. Therefore, Country, family, place, and name, Shape, form, time are not the same he has sufficiently discussed genus and species and has also explained what the highest genus and lowest species are, what the subalternate genera and species are, which things are species and genera at the same time, what the individual is, and how many meanings the terms ‘genus’ and ‘species’ have.

Chapter Four: About Difference

1. Difference is spoken of in three ways: commonly, properly, and most properly. Common difference is an otherness through which a thing in any way differs from itself or from another thing to which it is in some way related. For example, Socrates the man differs even from himself as a boy, as well as from Plato and all other boys. And since Socrates is sitting, he differs from himself walking, and from Plato and all other walkers. Therefore age, sitting, etc., and all other separable accidents are common differences of Socrates.

2. A proper difference is an accident inseparable from the thing, through which one thing differs from another. For example, blueness is a proper difference, since it is an accident inseparable from him whose eyes are blue, through which he differs from those who do not have blue eyes. A hooked nose, a scar from a wound, etc., are also proper differences for this reason.

3. The most proper difference is that through which one species differs from another. For example, rational is the most proper difference of man, since man through his rationality differs from the horse.

4. Common, proper, and most proper differences have in common that they make a thing be diverse in a certain way and make it exist in a diverse way. They differ from each other because common and proper differences do not make something be another thing, but only make it exist in another way, while the most proper difference makes it be another thing. Therefore we can divide differences by a two-membered division, saying that some differences only alter things, but others make something be another thing. Those which make something be another thing are specific. For example, adding the difference rational to animal makes man be another species of animal different from the horse. But adding the difference walking to a man who before was resting does not make him be another man, but only the same man existing in another way. The man walking and the man sitting are the same man, but he now exists in a way different from the way in which he exists when he sits. Thus the divisions of genera into their species are not made through differences which only alter something, but only through differences which make it be another thing. For example, if animal is to be divided into its species, it is not divided into ‘animals actually walking’ and ‘animals actually resting,’ but into ‘rational animal’ and ‘an animal which is able to neigh.’ Similarly, since species are defined through a genus and a difference, they are not defined through differences which only alter a thing, but through differences which make it be another thing. For example, man is not defined as an animal which is actually walking, but as the rational animal. Through the differences which only alter a thing, we can only explain how a thing exists; for example, whether a man walks or rests.

5. So that we might divide differences more exactly and better explain the nature of the specific difference which we intend to discuss, we must say that differences are first divided into the separable and the inseparable. For example, the differences of walking and resting, of sick and healthy, are separable from that of which they are the differences. For a man sometimes walks, sometimes rests; sometimes he is healthy, sometimes sick. But the differences of having an aquiline nose or of having a snub nose are inseparable from that of which they are the differences because he who has the aquiline nose is not able to change it into a snub nose. The differences rational and irrational are also inseparable, since no animal that is now rational is able to become irrational.

6. Second, inseparable differences are subdivided into essential and accidental differences. For example, the differences rational, mortal, and able to learn inhere in man essentially, but the differences of aquiline and snubnosed, although they are inseparable, are not essential, but only inhere in man accidentally. Therefore, only those differences which inhere essentially are taken in the notion of the substance, that is, in the definition explaining the essence of the thing, and only those make something be another thing. The differences which inhere accidentally are neither taken in the essential definition, neither do they make it be another thing, they only alter it. For example, rational is taken in the definition of man, since man is defined as the rational animal; but the difference of having a snub nose is not taken in the definition of man, since man is not defined as an animal with a snub nose.

Again, differences which inhere essentially do not admit of more or less, nor are they capable of being intensified or diminished. Inseparable differences which inhere accidentally, however, admit of more and less and are capable of being intensified or diminished.22 For example, one man can indeed have a more snub nose or a more aquiline nose than another, but one man is not able to be more rational than another. The reason is that the essence of a thing is not able to be intensified and diminished, but the genus and the difference constitute the essence of the thing. Therefore genera and differences are not capable of being intensified or diminished.

7. Third, essential differences are subdivided into constitutive and divisive. Constitutive differences are those which are predicated essentially of that which is divided, so that they constitute and specify it. Divisive differences are those which are predicated essentially of that which is divided, but do not constitute it. For example, the essential differences of animal are not only the sensitive and animate, but also the rational and irrational, because all of these are predicated of animal essentially. But animate and sensitive are the constitutive differences of animal, since animal is constituted from this, that a body is both animate and sensitive. Rational and irrational, in contrast, are not constitutive differences but only divisive differences, since animal is not constituted from rational and irrational but is only divided into species through them. The differences dividing genera, however, are constitutive of their species. For example, the differences rational and irrational, mortal and immortal, so divide the genus animal that rational and irrational constitute the species of man and the gods, according to the error of the Platonists, who thought that the gods, that is, angels, were rational immortal animals. Of course, irrational and mortal constitute the species of the brute animals. Similarly, the differences essentially dividing substance are animate
and inanimate, sensitive and insensitive. For if we add animate and sensible to substance, we constitute animal, but if we add animate and insensitive we constitute plant. Since, as we have explained, every difference is in one way divisive, in another way constitutive and specifying, then all these differences are called specific. And we most need such differences for dividing genera into species and defining the species, as was said above. But we do not need accidental differences, whether separable or inseparable, in any way, since genera are not divided into species through such differences, nor are species defined through accidental differences.

8. Having explained the divisions and subdivisions of difference, Porphyry now offers five descriptions of specific difference taken from the ancients. The first description is: specific difference is that through which a species overflows the genus, or that through which a species exceeds the genus. For example, rational is the difference of man, since man is not only an animal, but also rational. Therefore above animal, which is the genus, man adds the difference rational, since the genus animal, taken precisely in itself, is neither rational nor irrational.

But Porphyry objects against himself by saying that, if the genus does not contain the differences, how can it receive them? For example, if the genus animal does not contain the difference rational, how can man receive the difference rational? He replies that the genus does not contain the differences in act, but it does so in potency, just as matter contains form. Through this response two truths are preserved: first, that the genus does not have the opposite differences rational and irrational at the same time in act; second, that the differences do not come to be from nothing, but rather are drawn out of the potentiality of the genus.

9. He offers a second description of difference insofar as it is a predicable: difference is predicated of many differing in species in answer to the question “Of what kind?” For example, the differences rational and mortal are said of man, but not in answer to the question “What is it?” For the one asking “What is man?” is fittingly answered by being given the genus and being told that man is an animal. But he is not fittingly answered by being given the difference and being told that man is the rational, mortal animal. The reason is that, just as things are physically constituted from matter and form, they are metaphysically constituted from genus and difference, which are proportionate to matter and form. Therefore the genus stands as matter and the determinable part, while the difference stands as form and the part determining and qualifying the genus. Since it qualifies the genus, it answers the question “Of what kind?” For example, just as the statue is constituted from bronze as its matter and shape as its form, so cow is constituted from the genus animal as its matter, and the differences irrational and mortal as its form. In the definition of difference Porphyry follows the opinion of Plato, who said that simple and convertible differences are not to be given to species, but only differences more common than the species.

10. He offers a third description of difference by saying: difference is that which is apt by nature to divide those things which are under the same genus. For example, rational
and able to neigh are specific differences of animal, since they divide horse and man from each other,nand these are both under the genus animal.

11. He offers a fourth description of difference by saying: difference is that through which each thing differs. For example, rationality and the ability to neigh are the differences of man and horse, since man and horse, which belong together in the genus animal, differ through this, that man is the rational animal, but the horse is the animal able to neigh. Again, according to the error of the Platonists men and angels are not entirely different but have in common that they are rational animals. They do differ, however, through mortality and immortality. For the angels, according to their error, are rational immortal animals, while men are rational mortal animals.

12. Those who more intimately penetrate the nature of specific difference offer a fifth description which is more exact, saying that specific difference is not just anything
which divides those things in the same genus but is that which divides them by looking to the essence or “whatness” of each thing, and it is part of what makes one of them what it is. For example, although man is divided from the other animals through this, that while the other animals are not able to sail, man is an animal capable of the nautical art, the aptitude for sailing is not the specific difference of man. The aptitude for sailing does not look to the essence of man, but only to a certain property which follows upon rationality, which itself is the essence of man. Specific differences therefore are those which divide species and make them other, being parts which make each species what it is.

The fact that difference is a part making a species what it is ought to be understood as applying to all of the other descriptions which have been offered. For that in which the species exceeds the genus, and that which is predicable of many differing in species with respect to what each is, and that which is naturally able to divide those which are under the same genus, and that through which things differ, are not specific differences unless they are parts of what makes the species what it is.

Chapter Five: About Property

1. ‘Property’ has four meanings. First, property is that which happens only to some species, but not to all of the individuals of that species. For example, to be a physician or a geometer is proper to man, since only man is able to be a physician or a geometer, although not every man is a physician or a geometer.

2. Second, property is that which happens to all of a species, but not to that species alone. For example, to be two-footed is proper to man, since every man is twofooted, but not only man is two-footed, since birds also are.

3. Third, property is that which happens to all of a species and only that species, but not always. For example, becoming gray-haired in old age is a property of man, since all men and only men become gray-haired, but they are not always gray, but only sometimes, namely, in old age.

4. Fourth, property is that which happens to all of a species, only that species, and always. For example, to be able to laugh is proper to man, since at all times all men and only men are able to laugh. For while a man is not always actually laughing, still from his nature he is always capable of laughing, just as a horse is not always neighing but is always capable of it from its nature. -Property understood in this fourth way is the predicable, is strictly called property, and is convertible with a species. For just as every horse is able to neigh, so everything able to neigh is a horse.

Chapter Six: About Accident

1. Accident is that which is present or absent without the corruption of the subject. It is divided into separable and inseparable accident. For example, the act of sleeping is
an accident separable from man, since sleeping belongs to man in such a way that he retains the ability not to sleep. On the contrary, blackness is an accident inseparable
from the crow and the Ethiopian. From this arises the difficulty of whether the definition of inseparable accident fits with the definition of accident. For accident is defined as what can be present or absent without the corruption of the subject, but an accident is inseparable, for example the blackness of the crow or Ethiopian, when it cannot be separated unless the subject is corrupted. But the inseparable accident is able to be absent in the understanding, without the subject being corrupted, and this suffices for the definition.

2. Accident is also defined as that which happens to be or not to be, in the same thing. Again, it is defined as what is neither the genus, nor the species, nor the difference, nor the property, but which nevertheless subsists in a subject. Porphyry therefore concludes that we have now discussed all five predicables.


1 Silvester Maurus, Aristotelis Opera Omnia Illustrata, vol. 1 (Paris, 1885).
2 Bertrand Russell, Our Knowledge of the External World (London: George Allen and Unwin, Ltd., 1926), 42.
3 Ibid., 51.
4 Ibid., 55.
5 Ibid., 68.
6 Ibid., 69.
7 Ibid., 51.
8 Ibid., 51.
9 St. Thomas Aquinas, In Boethii De Trinitate, q. 5, a. 1.
10 Russell, 72.
11 St. Albert the Great, Commentary on the Isogoge of Porphyry, Tractate 1, Chapter 1.
12 St. Thomas Aquinas, Commentary on the Posterior Analytics of Aristotle, prologue, n. 1-2.
13 For St. Thomas’ discussion of the meaning of predication and the way in which universal words are predicated of the individuals below them, see On Being and Essence, c. 2.
14 Porphyry, Isogoge, Chapter 1.
15 Russell, 51.
16 St. Thomas Aquinas, Commentary on the Metaphysics of Aristotle, Book VIII, lect. 1, n. 1681.
17 Aristotle, On the Soul, 430a 26.
18 Chapter VII is omitted from this translation.
19 The English equivalent of ‘genus’ is ‘kind’ or ‘kindred.’
20 I have supplied an obvious omission in the printed text.
21 The sense of the text requires these phrases to be inserted, although they are not in the latest Latin edition.
22 Curiously, Descartes takes this scholastic principle as the starting point of his Discourse on Method (Part I).