This basic notion of Aristotle's logic reflects the basic distinction in the way reality is stuctured and reflects the basic way that we view reality. The fundamental distinction is between substance and accident. Substance is whatever is a natural kind of thing and exists in its own right.
Examples are rocks, trees, animals, etc.
What an animal is, a dog for example, is basically the same whether it is black or brown, here or there, etc. Substance and Accidents Accidents are the modifications that substance undergo, but that do not change the kind of thing that each substance is. Accidents only exist when they are the accidents of some substance. Examples are colors, weight, motion. For Aristotle there are 10 categories into which things naturally fall.