Immateriality of form is the principle of knowledge

From Nature, Knowledge and God: An Introduction to Thomistic Philosophy, by Bother Benignus, F.S.C., Ph.D.

Form and Intelligibility. Intelligibility means capacity to be understood or to be an idea. It is St. Thomas’ teaching that forms are of themselves intelligible; that is to say, they are ideas, either potential ideas, when they are immersed in matter, or actual ideas, when they are free from matter. The form of anything is its idea, and this form, in the mind, makes a thing known to the mind.

“What is called “idea” in the Greek is called “forma” in Latin. Whence by ideas we understand the forms of certain things existing apart from things. Now the form of a thing existing apart from the thing can have a twofold being: it may either be the exemplar of that of which it is called the form, or it may be the principle by which the thing is known, for which reason the forms of things known are said to be in the one knowing.” (S. theol., I Q. 15, Art. 1, C. Cf. De Ver., III, 2, c.)

When a form is not actually intelligible, this is not due to anything in itself as form, but to its mode of existence in matter. Every form existing free from matter is actually intelligible, that is to say, is an actual idea.

Form and Intelligence. St. Thomas goes further than this. He not only equates form with intelligibility, but also with intelligence. How far the Angelic doctor goes in identifying form with intelligibility and intelligence is made clear by the following passages:

“Just as matter is the principle of particularity, so is intelligibility due to form. For this reason form is the principle of knowledge. Wherefore, it follows necessarily that every form existing in itself apart from matter is intellectual in nature; and if, indeed, it subsists in itself, it will also be an intelligence. If, on the other hand, it is not subsistent but rather a perfection of some subsistent being, it will not be an intelligence, but a principle of understanding. (In I Sent., d. 35. Q. 1, 1, c.)

“If there were a box subsisting in itself without matter, it would understand itself; because of this, the box without matter would not differ from the intelligible box.” (De Spirit. Creat., I ad 12.)

“Every form subsisting in itself without matter is an intellectual substance; for immunity from matter confers intelligibility.” (Con. Gen., II, 91.)