Categories (or "Categoriae") is a text from Aristotle's Organon that enumerates all the possible kinds of thing which can be the subject or the predicate of a proposition.

The Categories places every object of human apprehension under one of ten categories (known to medieval writers as the praedicamenta). They are intended to enumerate everything which can be expressed without composition or structure, thus anything which can be either the subject or the predicate of a proposition.

The text begins with an explication of what is meant by "synonymous" or univocal words, what is meant by "homonymous", or equivocal words, and what is meant by "paronymous", or denominative words. What we say:

Is either simple, without composition or structure, such as "man", "horse", "fights", etc;

Has composition and structure, such as "a man fights", "the horse runs",

Next, we distinguish between a subject of predication, namely that of which anything is affirmed or denied, and a subject of inhesion. A thing is said to be inherent in a subject, when, though it is not a part of the subject, it cannot possibly exist without it, e.g., shape in a thing having a shape.

Of all the things that exist,

• Some may be predicated of a subject, but are in no subject; as "man" may be predicated of James or John, but is not in any subject.
  
• Some are in a subject, but can be predicated of no subject. Thus my knowledge in grammar is in me as its subject, but it can be predicated of no subject; because it is an individual thing.
  
• Some are both in a subject, and may be predicated of a subject, as science, which is in the mind as its subject, and may be predicated of geometry.
  
• Last, some things can neither be in a subject nor be predicated of any subject. These are individual substances, which cannot be predicated, because they are individuals; and cannot be in a subject, because they are substances.
  

Then we come to the categories themselves, (1)-(4) above being called by the scholastics the antepraedicamenta. Note, however, that although Aristotle has apparently distinguished between being in a subject, and being predicated truly of a subject, in the Prior Analytics these are treated as synonymous. This has led some to suspect that Aristotle was not the author of the Categories.

The ten categories, or classes, are

1. Substance. As mentioned above the notion of "substance" is defined as that which can be said to be predicated of nothing nor be said to be within anything. Hence, "this particular man" or "that particular tree" are substances. Later in the text, Aristotle calls these particulars "primary substances," to distinguish them from "secondary substances," which are universals. Hence, "Socrates" is a primary Substance, while "man" is a secondary substance.
   
2. Quantity. This is the spatial extension of an object. All medieval discussions about the nature of the continuum, of the infinite and the infinitely divisible, are a long footnote to this text. It is of great importance in the development of mathematical ideas in the medieval and late scholastic period.
   
3. Quality This is a determination which characterises the nature of an object.
   
4. Relation This is the way in which one object may be related to another.
   
5. Place (position in relation to the surrounding environment)
   
6. Time (position in relation to the course of events)
   
7. Posture (the relative position of the parts of the object (usually a living object)
   
8. Habit (the determination arising from the physical accoutrements of an object, such as clothed, driving &c)
   
9. Action (the production of change in some other object)
   
10. Passion (the reception of change from some other object).
    

The first six are given a detailed treatment in four chapters, the last four are passed over lightly, as being clear in themselves. Later texts by scholastic philosophers also reflect this disparity of treatment.

After discussing the categories, four ways are given in which things may be considered contrary to one another. Next, the work discusses five senses wherein a thing may be considered prior to another, followed by a short section on simultaneity. Six forms of movement are then defined: generation, destruction, increase, diminution, alteration, and change of place. The work ends with a brief consideration of the word 'have' and its usage.

Categories, translated by E. M. Edghill.