I think it is best to let Aquinas himself explain:
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
[Note: in Aquinas “exsistere” does not mean “to be,” but rather “to emerge” or (as in this case) “to be present.”] 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 as it is intelligible, 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 (Question 46, Article 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 (
I, q. 15, a. 1).
This is the article regarding the Divine Ideas (which shows that Aquinas is not at all a slavish interpreter of Aristotle).
Aquinas, as is his wont, makes an analogy with creaturely agents. Some things produce their effects because they possess a perfection really (like when hot coals set things on fire). However, more perfect creatures, like men, can produce effects because they have
conceived them in their minds, like when an architect builds (or designs) a house.
God’s causality is of the second kind. He does not have to possess the heat of the fire in order to create fire. He has the
idea of fire, and thus creates fire.
There is an important clarification to be made, however: although we can truly say there are many Divine Ideas, we can only say so because there are many creatures.
Creatures derive their perfections—their “act”—by participation in God’s perfection. They participate, naturally, in differing degrees, which explains the diversity of creatures. As Aquinas explains:
Inasmuch as He knows His own essence perfectly, He knows it according to every mode in which it can be known. Now it can be known not only as it is in itself, but as it can be participated in by creatures according to some degree of likeness. But every creature has its own proper species, according to which it participates in some degree in likeness to the divine essence. So far, therefore, as God knows His essence as capable of such imitation by any creature, He knows it as the particular type and idea of that creature; and in like manner as regards other creatures. So it is clear that God understands many particular types of things and these are many ideas (
I, q. 15, a. 2, c).
Said in simpler terms, every perfection found in any creature (even the impure perfections) are a participation in the infinite perfection of God. Hence, when God endows His creatures with such perfections, His own Essence is the Exemplar. That Essence, considered inasmuch as it is communicated to His creature, is called a “Divine Idea.”
Thank you, Imelahn. I have read the two articles. St. Thomas realized that his exposition would receive an objection concerning God’s simplicity and he offers a response; but it is not conclusive: to know something is a mode of being, and to know something in many different ways is a variety of modes of being. God would have a multiplicity of modes of being simultaneously, and so He would not be simple.
Concerning my question, these passages of the Summa do not respond to it. I asked “how can God be the exemplar of impure perfections?”, and St. Thomas just says that creatures participate to some degree in likeness to the Divine Essence; but to say something is not to explain it.
Consider God’s simplicity (not easy, I guess, because we have no experience of simplicity) when you conceive Him (impossible, I know) creating a creature. How can He, in His simplicity, be the exemplar of impure perfections? He is not being imitated by the thing; He is creating it.