Aristotle's Philosophy of Nature - Substance and Accident

Aristotle says all things of nature are composed of substance and accident. The term accident really means what we might call any feature of a thing as distinct from its natural kind. So if we had a dog here in front of us, Fido, and if I were to ask you what is this, if you were to say a dog, you're telling me the substance of the thing. That's how Aristotle thinks about things.