Principle E. A form is, by definition, an act
This principle indicates why Aquinas writes that a knower takes on the ‘form’ of an object in the external world. Structurally, the relation of form to act also applies to Aquinas’s ontology: ‘For, because of form, which is an actuation of matter, matter is made into an actual thing’ (De Ente et Essentia, lec. 2). The concept of form is connected intrinsically with the concept of the act/potency distinction. It is only upon realization of the latter distinction that the former concept can be understood.
-  Concerning this principle, it should be noted that this dos not entail that an act is identified with aform. Rather, a form is a type of act. That Aquinas would not make such an identification is apparent fromthe fact that in his ontology, as noted above, God is ‘Pure Actuality’—an Actus Purus—but not a form. Aform is a type of act, which needs a potential complement. The combination of this potential complementwith an act brings about an existent. Thus form and act are neither identical nor coextensive.