Aristotle's Substantial Change

Aristotle thinks that all the things around us are composed of substance and accident, and they're also composed of matter and form. When a thing has a dog form, it actually is a dog when it has the accidental form of being white or tall. That's what we mean by form. And form is the principle of actuality as distinct from potentiality. We could talk about that at great length in the Q&A if you'd like.