Aristotle’s Categories is an ontological piece attempting to differentiate between states of being. It is a short piece, broken up into fifteen chapters. The most basic component is the distinction between the subject and the predicate. The former is what the statement is about; the latter is what is describes.
In chapter two, Aristotle gives a two-type difference between the natures of the subject in a statement of truth. Firstly, there is a statement that which is said of the subject. This type of ontological deduction is that which is essential to the subject. It is arranged in universal hierarchies. One example would be “the saxophone is (said of) an instrument” where the “saxophone” is the more specific type than the lesser universal distinction of “instrument.” This statement of truth is essential to the subject. However, both “instrument” and “saxophone” are two distinct parts and can exist independently in different statements of truth.
The second differentiation deals with what is present in the subject. This signifies dependence, since it can not exist without the subject. Thus, it is description which is non-essential to that subject. Such an example would be “Aristotle was wise” where wisdom is used as a description of the subject. In this case, the description of “wise” cannot exist in the same context without its subject. Therefore, it is not a part all of itself; it is merely present in the subject.
This distinction forms the basis of Aristotle’s ontology since it differentiates between two states of being. That which is said of the subject are parts which are essential to the subject, but are not dependent on each other in statements of truth. And that which is present in the subject is a description of it which cannot exist without the subject.