Aristotle, in books Z, H, and Θ of "Metaphysics," examines what substance is, that is, the true foundation of every existence.

He concludes that every thing is composed of matter and form: matter is what can become something, while form is what makes it what it is.

In book Θ, he explains that beings exist first as potentiality (dunamis) and are completed as actuality (energeia). This shows how things come into being and what makes them truly beings.