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. All these distinctions are basically logical, but in a sense they reflect the structure of reality. One never finds any substance that we experience without some accidents, nor an accident that is not the accident of a substance. Every dog, for instance, has some color, place, size. Nevertheless, it is obvious that what a dog is is not the same as its color, or its size, etc.