Substantial form

A theory of substantial forms asserts that forms (or ideas) organize matter and make it intelligible. Substantial forms are the source of properties, order, unity, identity, and information about objects.

The idea of substantial forms dominates ancient Greek philosophy and medieval philosophy, but has fallen out of favour in modern philosophy. The idea of substantial forms has been abandoned for a mechanical, or “bottom-up” theory of organization. However, such mechanistic treatments have been criticized for the same reasons atomism has received criticism, viz., for merely denying the existence of certain kinds of substantial forms in favor of others (here, that of atoms, which are then thought to be arranged into things possessing accidental forms) and not denying substantial forms as such, an impossible move.

Plato maintains in the Phaedo regarding our knowledge of equals:

Aristotle was the first to distinguish between matter (hyle) and form (morphe). For Aristotle, matter is the undifferentiated primal element: it is rather that from which things develop than a thing in itself. The development of particular things from this germinal matter consists in differentiation, the acquiring of particular forms of which the knowable universe consists (cf. Formal cause). The perfection of the form of a thing is its entelechy in virtue of which it attains its fullest realization of function (De anima, ii. 2). Thus the entelechy of the body is the soul. The origin of the differentiation process is to be sought in a prime mover, i.e. pure form entirely separate from all matter, eternal, unchangeable, operating not by its own activity but by the impulse which its own absolute existence excites in matter.

...
Wikipedia