CLAA – TRV-371 - Porphyry, Introduction, Chapter 1

Classical Liberal Arts Academy
Mr. William C. Michael, O.P.
05/07/2026

Lesson Text: Porphyry, Introduction. Chapter 1

Introduction

Welcome to the Classical Liberal Arts Academy. This lecture is intended for students enrolled in the Academy, who are studying according to the instructions provided on “How to Study for Mastery” at classicalliberalarts.com/mastery. Courses in the Classical Liberal Arts Academy provide students with access to all necessary study materials, graded assessments, records and expert support. If you would like to join us for self-paced classical studies, please visit classicalliberalarts.com.

“Lord our God, in your wisdom and love you surround us with the mysteries of the universe. Send your Spirit upon us and fill us with your wisdom and blessings. Grant that we may devote ourselves to our studies and draw ever closer to you, the source of all knowledge. We ask this through Christ our Lord. Amen.”

Prelection

In this lesson, we will study Porphyry’s Introduction, Chapter 1

Porphyry was born about AD 234 in Tyre, a city of Phoenicia. He was educated in rhetoric and philosophy, and later came to Rome where he became a disciple of Plotinus, under whom he advanced deeply in the study of philosophy. He devoted his life to study, teaching, and writing, and is especially known for editing and arranging the works of his master, as well as composing many treatises of his own, including the Isagoge. He died about AD 305, after a life given to philosophical inquiry, and is chiefly remembered as a principal transmitter of Neoplatonism and as the author of a work that served as a foundation for logical studies throughout the medieval schools.

Porphyry’s Introduction (Isagoge) was written as a preparation for the study of Aristotle’s Categories and serves as one of the clearest introductions to the basic ideas of logical thought. In it, Porphyry explains the five predicables—Genus, Species, Difference, Property, and Accident—which help students understand how we define and classify things. Throughout the Middle Ages, this work was studied by Catholic students as the first step in the study of logic and philosophy. Its importance for classical Catholic students lies in the fact that it trains the mind to think clearly, distinguish carefully between different kinds of ideas, and reason according to reality rather than confusion or opinion. By studying Porphyry, students begin to develop the intellectual habits needed for later studies in Aristotelian philosophy, Scholastic theology, and the liberal arts.

The objective of this lesson is to introduce the five basic ideas used in Aristotelian logic—Genus, Species, Difference, Peculiarity, and Accident—which are necessary for making definitions, divisions, and logical demonstrations. Porphyry explains that he will give only a simple and introductory treatment of these topics, avoiding deeper metaphysical questions about whether universals exist in reality or only in the mind. His purpose is to prepare students for the study of logic by teaching the fundamental ways things are classified and understood.

This lesson is historically significant for students of classical Catholic education because Porphyry’s Introduction (Isagoge) became the standard introduction to Aristotle’s logic throughout the Middle Ages and was studied for centuries in Catholic schools and universities. It helped form the intellectual foundation of Scholastic education by teaching students how to classify beings, form precise definitions, and reason logically. Great Catholic thinkers such as St. Thomas Aquinas studied within this logical tradition, using these tools in theology, philosophy, and science. The work also introduced the famous problem of universals, which later became one of the central philosophical debates of medieval thought. Thus, this lesson stands at the beginning of the classical Catholic tradition of disciplined reasoning and scientific inquiry.

Text

1. Οὖτος ἀναγκαίου, Χρυσαόριε, καὶ εἰς τὴν τῶν παρ’ Ἀριστοτέλει κατηγοριῶν διδασκαλίαν τοῦ γνῶναι τί γένος καὶ τί διαφορὰ τί τε εἶδος καὶ τί ἴδιον καὶ τί συμβεβηκός, εἰς τε τὴν τῶν ὁρισμῶν ἀπόδοσιν καὶ ὅλως εἰς τὰ περὶ διαιρέσεως καὶ ἀποδείξεως χρήσιμης οὔσης τῆς τούτων θεωρίας, σύντομόν σοι παράδοσιν ποιούμενος πειράσομαι διὰ βραχέων ὥσπερ ἐν εἰσαγωγῆς τρόπῳ τὰ παρὰ τοῖς πρεσβυτέροις ἐπελθεῖν, τῶν μὲν βαθύτερων ἀπεχόμενος ζητημάτων, τῶν δ’ ἁπλουστέρων συμμετρως στοχαζόμενος·

2. αὐτίκα περὶ τῶν γενῶν τε καὶ εἰδῶν τὸ μὲν εἴτε ὑφέστηκεν εἴτε καὶ ἐν μόναις ψιλαῖς ἐπινοίαις κεῖται εἴτε καὶ ὑφεστηκότα σώματά ἐστιν ἢ ἀσώματα καὶ πότερον χωριστὰ ἢ ἐν τοῖς αἰσθητοῖς καὶ περὶ ταῦτα ὑφεστῶτα, παραιτήσομαι λέγειν βαθύτατης οὔσης τῆς τοιαύτης πραγματείας καὶ ἄλλης μείζονος δεομένης ἐξετάσεως·

3. τὸ δ’ ὅπως περὶ αὐτῶν καὶ τῶν προκειμένων λογικώτερον οἱ παλαιοὶ διέλαβον καὶ τούτων μάλιστα οἱ ἐκ τοῦ περιπάτου, νῦν σοι πειράσομαι δεικνύναι.

Division of Text

The Purpose of Porphyry’s Introduction
“Since it is necessary…”
The Scope of Porphyry’s Intoduction
“In a summary way…”
The Sources of Porphyry’s Introduction
“In what manner, however, the ancients…”

Exposition

Of what-is-necessary, Chrysaorius, both for the of-the from-Aristotle Categories teaching, for the to-know what genus and what difference and both what species and what property and what accident, and for the of-the definitions rendering and wholly for the things concerning division and demonstration, useful being the of-these contemplation, a concise for-you exposition making, I-shall-attempt through brief things just-as in introductory manner the things from the elders to-go-through, from the on-the-one-hand deeper inquiries abstaining, but from-the other simpler ones proportionately aiming-at;

Porphyry opens his Introduction and explains that he intends to provide an essential and concise overview intended for a student named Chrysaorius, preparing him for the study of Aristotle’s Categories. Porphyry explains that it is necessary to understand fundamental concepts such as genus, difference, species, peculiarity, and accident, because these are key to grasping how definitions are made, and how division and demonstration operate in logic and philosophy. He promises to present this material briefly and in an introductory manner, avoiding the deeper and more difficult questions, while focusing on the simpler and more essential points. This approach is designed to prepare the learner for the more complex discussions by first establishing a clear understanding of these basic terms and their logical relationships. In this way, Porphyry offers a systematic foundation for the study of Aristotelian categories and the theory of classification, which is central to logical reasoning and philosophical inquiry.

autíka perì tôn genôn te kaì eidôn tò mèn eíte hyphéstēken eíte kaì en mónais psilaîs epinoíais keîtai eíte kaì hyphestēkóta sṓmatá estin ḕ asṓmata kaì póteron khōristà ḕ en toîs aisthētoîs kaì perì taûta hyphestôta, paraitḗsomai légein bathytátēs oúsēs tês toiaútēs pragmateías kaì állēs meízonos deoménēs exetáseōs·

tò d’ hópōs perì autôn kaì tôn prokeiménōn logikṓteron hoi palaioì dielábon kaì toútōn málista hoi ek toû peripátou, nûn soi peirásomai deiknýnai.

Part 1. The Purpose of Porphyry’s Introduction

1. Since it is necessary, both to the doctrine of the Categories of Aristotle, and to the formation of definitions, and in short, to those things which pertain to division and demonstration, to know what Genus and Difference, Species, Peculiarity, and Accident are; and since also the theory of these is useful:

In this opening sentence, Porphyry identifies for us the challenges that make this Introduction necessary.

First, in Aristotle’s Categories, which is the first book of Aristotle’s series on the art of reasoning, he shows that all of the things we say (predicate) about subjects can be divided into “ten Categories”: Substances, Quantities, Qualities, Relatives, Places, Times, Situations, Possession, Action, and Passion. Many students struggle to understand what Aristotle is actually teaching in the Categories, which leads to confusion and false ideas in reasoning and philosophy. The first problem is that modern students think of the word “categories” as if it means “classes” or “groups”, but the Greek word “katēgoriai” actually means “predicates”. The ten Categories are the ten highest genera of predicates, under which are many different species of predicates, as well as differences, peculiarities and accidents. They are not simply “classes” of predicates, but they are predicates themselves.

As you read that, you may be thinking, “I don’t really know what he’s talking about here. Genera? Species? Differences? Peculiarities? Accidents? I don’t know what these things are. How am I supposed to understand what Aristotle is talking about?”

Now, look at what Porphyry says: “Since it is necessary…to the doctrine of the Categories of Aristotle to know what Genus and Difference, Species, Peculiarity, and Accident are…”. You can see that this is exactly what Porphyry intends to explain for us in the Introduction. The problem we have run into is that the ten Categories explain what we predicate about subjects, but they don’t explain the different modes (ways) of predicating. If we don’t understand the modes of predication, or ways in which we speak about subjects, we can’t understand what Aristotle is teaching in the Categories. To clear this up, Porphyry explained that there are five modes of predication, which are called the “five Predicables”: Genus, Difference, Species, Peculiarity and Accident. The purpose of his Introduction is to teach us exactly what these are so that we can go on to study Aristotle’s Categories with understanding.

Second, Porphyry says that studying the Categories isn’t the only reason to learn about the five Predicables. Another important skill in the study of reasoning and philosophy is understanding definition. When most people attempt to give definitions of terms, they simply describe them. Descriptions are not definitions. If you were asked, “What is a human?”, you could answer, “A human is someone who lives in houses, wears clothes, talks with other people, and uses tools.” That would be a good description, but it wouldn’t explain exactly what a human is. Is a newborn baby a human? Is a homeless person a human? Is a person who can’t talk a human? The definition of a human has to explain exactly what a human is, and it has to be true of all humans at all times. If we try to define human, we’ll find it’s very difficult. Fortunately, there is a rule for making definitions. To define something, we must give two important pieces of information about it. We must name the genus to which it belongs and we must name the difference that sets it apart from all other things in that genus. So, in order to make definitions, we have to understand what Genus and Difference are — and they are two of the five Predicables. Porphyry is going to teach this in his Introduction.

Third, Porphyry says that another important skill in reasoning and philosophy is division. St. Thomas Aquinas explained that it is the business of a wise man to know the order in things. A wise man needs to know the order of his own soul, of his own will and of the world around him. It is not enough to simply collect information about all of these things. We must study to know the order that God created in all things and we are responsible to help keep things in that order. The knowledge of this order is called “Wisdom” and the pursuit of wisdom is called “Philosophy”. For us to know the order in things, we must know how different things relate to one another. When we think about different things, we can see that many are similar in some ways and different in others. If we look at a man and a woman, they are both humans. In their bodies, they are similar in many ways: they have two legs, two arms, five senses, a mind, and so on. Yet, there are also differences in their bodies. If we look at two men, we see that there are similarities and differences between them as well. If we look at a man and a dog, we see that there are similarities and differences between them. If we look at a man and a plant, we can see similarities and differences. Even if we look at a man and a rock, we can find similarities and differences. We can observe all of these similarities and differences, but how can we make any sense of the order of them? Porphyry is going to explain that knowledge of the five Predicables helps us to know this order that exists in things. When we’re finished with Porphyry’s Introduction, you won’t be able to know this order yet — we have to study Aristotle’s works for that — but you will understand what you need to know.

Lastly, Porphyry says that to study Aristotle’s philosophy well, we will need to understand demonstration. By demonstration, he means the ability to prove that something is true or false. Above, we saw that it is difficult to make definitions, even though many people think they can do so. The same thing is true when we’re asked to prove that something is true or false. It’s not enough to show that something may be true or false. In philosophy, we must prove with absolute certainty that a thing is true or false. Imagine if you met a man who told you that he is not going to die. How would you prove to him that he’s wrong? You could argue that everyone who has ever lived has died, but he would not be convinced by that. The fact that all other men have died does not prove that he is going to die. He could be the first man to not die. Your argument may show that he will probably die, but it did not show that he will certainly die. As with making definitions, it turns out that demonstration is difficult as well. Fortunately, Aristotle will teach us how to make demonstrations that prove things are true or false with certainty, but you will need to understand how to form propositions in which you predicate something about a subject that truly identifies what it is. In order to do this, you need to understand the different modes of predication, that is, the five Predicables. In his Introduction, Porphyry is going to help you get started.

So, in this opening sentence, Porphyry explained that there are at least four reasons why we need to understand the five Predicables if we wish to become wise. They are easy to remember because they all begin with the letter “D”. We need to understand the five Predicables so that we can prepare to study: (1) the doctrine of Aristotle’s Categories, (2) definition, (3) division and (4) demonstration.

Part 2. The Scope of Porphyry’s Intoduction

2. In a summary way, I will briefly endeavour to discuss for you, in the form, as it were, of an Introduction, what has been delivered on this subject by the ancients, abstaining from more profound investigations, but appropriately directing my attention to such as are more simple.

After providing the reasons why the knowledge of the five Predicables is to be desired, Porphyry explains his intentions for this Introduction. He makes it clear that will present a summary of what ancient wise men have taught on the five Predicables, going only as far as will be helpful to provide the benefits he has described. More specifically, he explains that he is only interested in teaching what pertains to the study of the art of reasoning and will not make any attempt at a complete study of the five Predicables. When he says that he will be “abstaining from more profound investigations” he makes it clear that he will not allow this discussion to run into questions appropriate for the science of Metaphysics, which is to be studied later in Aristotle’s works. His purpose here is to provide an introduction for Logic students, and he will keep things simple.

3. My meaning is, that I shall omit to speak about Genera and Species, whether they have a subsistence in the nature of things or have an existence alone in the mere conceptions of the soul; and if they have a subsistence in the nature of things whether they are bodies or incorporeal, and whether they are separate from sensibles, or in sensibles, and about these have their subsistence.

Here, Porphyry explains exactly what he means when he says that he will be “abstaining from more profound investigations” by providing a list of questions that would be appropriate in a Metaphysics course, but not an introductory Reasoning course. His goal in this Introduction will be to define each of the five Predicables and to compare and contrast them that students may gain a clear understanding of each of the modes of predication: what each is and what each is not; what they have in common and how they differ. Having said that, it is helpful for us to read of some of the “profound investigations” that will not be discussed here but await us in future philosophical studies.

First, Porphyry says that he will not be discussing “whether Genera and Species have a subsistence in the nature of things or have an existence alone in the mere conceptions of the soul”. This is a question that would divide followers of Plato from the followers of Aristotle. “Genera” is not a new term, but is simply the plural form of Genus. If I close my eyes and think about myself, I can see an image of myself in my mind, and I can say my name quietly — “You are William C. Michael” — but I know that the idea in my mind and the name in my mouth are symbols of the real me that exists outside of my mind in the natural world. I don’t simply think I exist, and I didn’t simply make up a name for the idea of myself in my mind — I really exist outside of my mind. On this Plato and Aristotle would agree. Something interesting happens, however, when I speak of a group to which I belong. If I say — “You are a man.” — is the same thing true? Does “man” exist outside of my mind, in the natural world? Or, is “man” only a name for an idea that exists in my mind? What if I say, “You are an animal.”? Is “animal” something that actually exists outside of my mind, in the natural world? Or, is “animal” only a name for an idea that exists in my mind? On this question, Plato and Aristotle would disagree. Aristotle would argue that only individuals exist in the natural world, outside of our minds. Our minds can recognize different things that are common in individuals and we can speak of them, but they do not exist as separate things themselves in the natural world. For example, if I look at myself, my son and my neighbor and say “All of us individuals are men.”, Aristotle would explain that something called “man” does not exist in the world outside of us, but that there are three beings present and the idea “man” exists in them (and in many others). Plato would disagree. He would argue that the three individuals and “man” exist separately as four beings. Plato would say that “man” does actually exist outside of our minds and apart from the individuals whom we say are “men”. Plato would argue that when we look at individuals and say that they are “men”, we are saying that they are like “man”, which exists apart from them. This complicated problem is not for us to settle here, but to study later in Metaphysics.

Second, Porphyry says that he also will not discuss the question that arises when, if we assume that Genera and Species do “have a subsistence in the nature of things”, “whether they are bodies or incorporeal”. If Plato was right, and Genera and Species did exist outside of our minds, a new problem would arise: do Genera and Species exist as beings with material bodies or as beings without any material bodies? Does “man” actually have size and shape, arms and legs, height and weight? Or does “man” exist as some kind of spiritual being? If “man” has no body, how could any individuals living in the world be like “man”? If “man” did have a body, would it be a perfect body? What would such a body look like? What would we think of individuals who did not look exactly like “man”? These would be very difficult questions that we could not answer as we’re just getting started in the study of reasoning. Porphyry knows we should not waste our time on such questions now, but save them for a later time in our studies.

Third, Porphyry says that he will also not discuss the question we face when, if we assume that Genera and Species exist outside the mind, whether they are (1) “separate from sensibles” or (2) “in sensibles and about these have their subsistence”. This is related to the first question we discussed above, but seeks to understand how Genera and Species would exist outside the mind. If Plato was right and the species “man” does exist outside of the mind, where does it exist? Is it separate from all physical things in some other world? Or, is it here with us among physical things? If Aristotle was right and the species “man” exists outside of mind, but in individuals, how exactly does it exist? These, again, are very complicated problems that we do not have the wisdom to answer. Porphyry, being a wise man and an excellent teacher, knows this and will not trouble us with such problems when we need to focus on preparing to study Aristotle’s works on the art of reasoning first.

4. For a discussion of this kind is most profound, and requires another, and a greater investigation.

Having mentioned the kinds of questions that ought not to be discussed in an introductory study of the five Predicables, Porphyry explains that these should not be ignored but that they should be saved for an appropriate time and place, when students have the knowledge and skills necessary to investigate them effectively. The “greater investigation” that Porphyry refers to is the science of Metaphysics. If you look at the list of courses in the Classical Liberal Arts Academy, you will see different subject areas: Classical Physics, Classical Mathematics (the Quadrivium) and Classical Metaphysics. Each of these investigates a different kind of “being”. In Physics, we study material beings that exist in motion. In Metaphysics we study being itself and immaterial beings. In Mathematics, which lies in between Physics and Metaphysics, we study things that can be understood apart from material things, but that exist in material things. Metaphysics is the most advanced of these three sciences, which we study last. It is there, in Metaphysics, where we will study to answer questions about how Genera, Species and the other Predicables exist. So, by passing over these questions now, we are not ignoring them. We will study them in due time.

Part 3. The Sources of Porphyry’s Introduction

5. In what manner, however, the ancients, and especially the Peripatetics discussed these, and the other proposed objects of enquiry, in a more logical manner, I will now endeavour to show you.

Having announced his intentions for his Introduction, and set the limits for the content of this study, Porphyry now explains what the sources will be from which he draws his instruction. He says that he will teach us the way that “the ancients” discussed the five Predicables, the doctrine of Aristotle’s Categories, definitions, division and demonstration. Remember that Porphyry lives from 234-305 AD, so when he refers to “the ancients”, he’s referring to philosophers who lived before him. Porphyry, would not be introducing false ideas from men whose teaching had been rejected by wise men, but teaching that he considered true and beneficial to his students. Porphyry was a follower of Plato’s teaching, so we can be sure that when he spoke of “the ancients”, he was referring to Plato, who lived from 427-347 BC and died almost 600 years before Porphyry was born. We can also be sure that Porphyry would include the teaching of Aristotle (384-322 BC) who was a student of Plato originally but grew to disagree with his teacher on several important issues and separated from him, as we saw above. Moreover, Porphyry mentions a specific group of ancient philosophers, whom he calls “the Peripatetics”. By this name he refers to those who followed the teaching of Aristotle. These men were called “Peripatetics” because Aristotle’s school in Athens, known as the Lyceum, was built with covered walkways (Greek, peripatoi) where he and his students could walk about while they discussed their studies. The name literally means “those who walk about”. So, in Porphyry’s Introduction, we will prepare for the study of Aristotle’s Categories but studying what the ancients — Plato, Aristotle and their followers — taught about the five Predicables and the subjects relating to them.

Porphyry mentions that the Peripatetics discussed these subjects “in a more logical manner” than other ancient sources, but this does not mean that the other philosophers were not reasonable. It’s important to note that before Aristotle, the art of reasoning was not understood in a systematic way. In his work called Organon, which means “the Method”, Aristotle taught the art of reasoning to the world. Though his books on the philosophical sciences have been famous for thousands of years, Aristotle’s greatest achievement was the discovery of the art of demonstrative reasoning. Before Aristotle, wise men like Socrates and Plato would discuss controversial subjects with others and prove that their opinions were impossible, but they could not establish positive scientific works that taught the truth persuasively. This could not be done until after the art of demonstrative reasoning was established, which Aristotle taught in his works on Analytics. Once Aristotle established the method by which these investigations could be carried out effectively, he went on to establish all of the philosophical sciences in his “logical manner”. His followers imitated his method and it is this approach to philosophical study that Porphyry refers to here.

Memory

The following sentences summarize the key points of this lesson and are recommended for memorization.

The five Predicables to be studied in Porphyry’s Introduction are: Genus, Difference, Species, Peculiarity, and Accident.
Knowledge of the five Predicables is necessary for understanding the doctrine of Aristotle’s Categories.
Knowledge of the five Predicables is essential for making definitions, divisions, and demonstrations.
In his Introduction, Porphyry intends to give a simple, introductory treatment of the five Predicables, and to avoid metaphysical questions about the nature of universals.
The sources of Porphyry’s Introduction include Plato, Aristotle, and their followers.

Conclusion

In this lesson, we learned that Porphyry wrote his Introduction to prepare students for the study of Aristotle’s works on reasoning. He explained that knowledge of the five Predicables is necessary for understanding Aristotle’s Categories, as well as for making definitions, divisions, and demonstrations. Porphyry also explained that his Introduction is only a simple and introductory treatment of these subjects, and that deeper questions about the nature of universals belong to the science of Metaphysics and must be studied later. Finally, Porphyry explained that his teaching follows the instruction of the ancient philosophers, especially Aristotle and the Peripatetics, who studied these subjects in a systematic and logical manner.

Support

We hope that you have found this exposition helpful. If you have any questions about this lesson or need help, please contact us through the Academy support center at classicalliberalarts.com/support.

Notes:

“Genera” is the plural form of Genus. The genitive singular form of the Latin noun genus is generis, which gives s the stem gener-.