Porphyry, Introduction. Chapter 1

Since it is necessary, 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 Accident1 are; and since also the theory of these is useful, 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.

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. For a discussion of this kind is most profound, and requires another, and a greater investigation2. In what manner, however, the ancients, and especially the Peripatetics3 discussed these, and the other proposed objects of enquiry, in a more logical manner, I will now endeavour to show you.


by Mr. William C. Michael

In the 5th century BC, ideas of democracy were growing in Greece and wise men saw a danger growing alongside them. As power moved into the hands of the people, opportunities opened for cunning men to deceive. These men, called “Sophists”, pretended to be wise in order to gain influence in society, and the common people could not detect their tricks. Wise men knew they were wrong, but couldn’t identify or explain what it was that was wrong.

Socrates made the first breakthrough, developing the practice which would later be known as “dialectical reasoning.” The “Socratic Method” was a helpful first line of defense, but it was not the solution. It could be used to prove that a man’s responses led to contradictions and were, therefore, impossible, but it could not prove, positively, what was true. Plato did nothing more, but continued with the Socratic method in his dialogues. It was Aristotle who provided the solution, establishing the art and science of Rational Philosophy. He published this art, in its entirety, in his work titled “Organon”, which means “the Method”.

The Organon

In the Organon, Aristotle published six books, each of which addressed a different part of the art of Reasoning. In the first book, the Categories, he discussed the rold of words in reasoning. In the second book, On Interpretation, he discussed propositions, and proved that they are the only acceptable mode of speech to be allowed in reasoning. In the third and most important book, “Prior Analytics”, he taught the doctrine of the syllogism, and how demonstrative reasoning actually works. In his fourth book, “Posterior Analytics”, he taught how the syllogism is to be employed for the development of philosophical sciences. In his fifth book, “Topics”, he systematized the “Socratic Method”, teaching the art of dialectical reasoning. In his sixth book, “Sophistical Arguments”, which brought the study full circle, he exposed the false arguments used by the Sophists to deceive unskilled hearers. These six books, taken together form the “Organon”, and are the source materials for our studies in Classical Reasoning:

  • Classical Reasoning I: Categories, On Interpretation
  • Classical Reasoning II: Prior Analytics
  • Classical Reasoning III: Posterior Analytics
  • Classical Reasoning IV: Topics
  • Classical Reasoning V: Sophistical Arguments

The Introduction

When we open lesson 01 of Classical Reasoning I, however, we do not find Aristotle’s Categories waiting for us. Instead, we find a book titled the “Introduction”. This book was written just before 300 AD to help students prepare for the study of Aristotle’s Organon. The author was a teacher of Platonic philosophy, named Porphyry. Porphyry had learned that students struggled to make progress in their study of Aristotle’s Categories, and in philosophy in general, because they did not have a clear understanding of five important concepts.

Chapter 1

As you begin the study of Porphyry’s Introduction, you will find that Porphyry speaks of three important things.

First, he identifies the five confusing concepts:

  1. Genus
  2. Difference
  3. Species
  4. Peculiarity
  5. Accident

Second, he explains that these can be very deep subjects, but he is not going to get into complex discussions about them. He says that the appropriate place for these discussions is in the study of Metaphysics. He will simply explain the meaning of these concepts as is necessary for the study of Aristotle’s Categories.

Third, he says that the source of his information on these five concepts will be the writings of the ancient philosophers. He will seek to allow the ancient philosophers to explain the meaning of these terms because we need to understand them in their original context, not as they are used in our own times by other men. More specifically, he says that he is going to stick to the teaching of the Peripatetics, that is, the followers of Aristotle.


With this lesson, we begin our study of Classical Reasoning. We will study the six books of the Organon of Aristotle, beginning with the Categories and On Interpretation in Classical Reasoning I. Before starting the Categories, however, we will study Porphyry’s Introduction, written in the 3rd century AD. In this lesson, we study the first lesson, in which Porphyry names the five concepts that need to be understood, what we do not need to discuss at this time, and where the source of our information should come from. Now, it’s time for you to study chapter 1 of the Introduction on your own. Make sure you follow the instructions I provide in my article “How to Study for Mastery“. If you have any questions or need any help, please contact me.

God bless your studies,
William C. Michael, Headmaster
Classical Liberal Arts Academy

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  1. Genus, Difference, Species, Peculiarity and Accident are called the five “predicables”, or classes of things that may be predicated of a subject.
  2. Aristotle refers here to the study of Metaphysics.
  3. The Peripatetics were the followers of Aristotle.