Objection 1. It seems that there are no ideas. For Dionysius says (Div. Nom. vii), that God does not know things by ideas. But ideas are for nothing else except that things may be known through them. Therefore there are no ideas.
Objection 2. Further, God knows all things in Himself, as has been already said (14, 5). But He does not know Himself through an idea; neither therefore other things.
Objection 3. Further, an idea is considered to be the principle of knowledge and action. But the divine essence is a sufficient principle of knowing and effecting all things. It is not therefore necessary to suppose ideas.
On the contrary, Augustine says (Octog. Tri. Quaest. qu. xlvi),"Such is the power inherent in ideas, that no one can be wise unless they are understood."
I answer that, It is necessary to suppose ideas in the divine mind. For the Greek word Idea is in Latin "forma." Hence by ideas are understood the forms of things, existing apart from the things themselves. Now the form of anything existing apart from the thing itself can be for one of two ends: either to be the type of that of which it is called the form, or to be the principle of the knowledge of that thing, inasmuch as the forms of things knowable are said to be in him who knows them. In either case we must suppose ideas, as is clear for the following reason:
In all things not generated by chance, the form must be the end of any generation whatsoever. But an agent does not act on account of the form, except in so far as the likeness of the form is in the agent, as may happen in two ways. For in some agents the form of the thing to be made pre-exists according to its natural being, as in those that act by their nature; as a man generates a man, or fire generates fire. Whereas in other agents (the form of the thing to be made pre-exists) according to intelligible being, as in those that act by the intellect; and thus the likeness of a house pre-exists in the mind of the builder. And this may be called the idea of the house, since the builder intends to build his house like to the form conceived in his mind. As then the world was not made by chance, but by God acting by His intellect, as will appear later (46, 1), there must exist in the divine mind a form to the likeness of which the world was made. And in this the notion of an idea consists.
Reply to Objection 1. God does not understand things according to an idea existing outside Himself. Thus Aristotle (Metaph. ix) rejects the opinion of Plato, who held that ideas existed of themselves, and not in the intellect.
Reply to Objection 2. Although God knows Himself and all else by His own essence, yet His essence is the operative principle of all things, except of Himself. It has therefore the nature of an idea with respect to other things; though not with respect to Himself.
Reply to Objection 3. God is the similitude of all things according to His essence; therefore an idea in God is identical with His essence.
Source: Summa Theologica (New Advent)