Reality is form.

And do you not know that they [the geometers] make use of the visible forms and talk about them, though they are not of them but of those things of which they are a likeness, pursuing their inquiry for the sake of the square as such and the diagonal as such, and not for the sake of the image of it which they draw. And so on in all cases... What they really seek is to get sight of those realities which can be seen only by the mind.
— Plato, Republic, VII, 510 d, e.