Objection 1. It seems that ideas are not many. For an idea in God is His essence. But God's essence is one only. Therefore there is only one idea.
Objection 2. Further, as the idea is the principle of knowing and operating, so are art and wisdom. But in God there are not several arts or wisdoms. Therefore in Him there is no plurality of ideas.
Objection 3. Further, if it be said that ideas are multiplied according to their relations to different creatures, it may be argued on the contrary that the plurality of ideas is eternal. If, then, ideas are many, but creatures temporal, then the temporal must be the cause of the eternal.
Objection 4. Further, these relations are either real in creatures only, or in God also. If in creatures only, since creatures are not from eternity, the plurality of ideas cannot be from eternity, if ideas are multiplied only according to these relations. But if they are real in God, it follows that there is a real plurality in God other than the plurality of Persons: and this is against the teaching of Damascene (De Fide Orth. i, 10), who says, in God all things are one, except "ingenerability, generation, and procession." Ideas therefore are not many.
On the contrary, Augustine says (Octog. Tri. Quaest. qu. xlvi), "Ideas are certain principal forms, or permanent and immutable types of things, they themselves not being formed. Thus they are eternal, and existing always in the same manner, as being contained in the divine intelligence. Whilst, however, they themselves neither come into being nor decay, yet we say that in accordance with them everything is formed that can rise or decay, and all that actually does so."
I answer that, It must necessarily be held that ideas are many. In proof of which it is to be considered that in every effect the ultimate end is the proper intention of the principal agent, as the order of an army (is the proper intention) of the general. Now the highest good existing in things is the good of the order of the universe, as the Philosopher clearly teaches in Metaph. xii. Therefore the order of the universe is properly intended by God, and is not the accidental result of a succession of agents, as has been supposed by those who have taught that God created only the first creature, and that this creature created the second creature, and so on, until this great multitude of beings was produced. According to this opinion God would have the idea of the first created thing alone; whereas, if the order itself of the universe was created by Him immediately, and intended by Him, He must have the idea of the order of the universe. Now there cannot be an idea of any whole, unless particular ideas are had of those parts of which the whole is made; just as a builder cannot conceive the idea of a house unless he has the idea of each of its parts. So, then, it must needs be that in the divine mind there are the proper ideas of all things. Hence Augustine says (Octog. Tri. Quaest. qu. xlvi), "that each thing was created by God according to the idea proper to it," from which it follows that in the divine mind ideas are many. Now it can easily be seen how this is not repugnant to the simplicity of God, if we consider that the idea of a work is in the mind of the operator as that which is understood, and not as the image whereby he understands, which is a form that makes the intellect in act. For the form of the house in the mind of the builder, is something understood by him, to the likeness of which he forms the house in matter. Now, it is not repugnant to the simplicity of the divine mind that it understand many things; though it would be repugnant to its simplicity were His understanding to be formed by a plurality of images. Hence many ideas exist in the divine mind, as things understood by it; as can be proved thus. 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.
Reply to Objection 1. The divine essence is not called an idea in so far as it is that essence, but only in so far as it is the likeness or type of this or that thing. Hence ideas are said to be many, inasmuch as many types are understood through the self-same essence.
Reply to Objection 2. By wisdom and art we signify that by which God understands; but an idea, that which God understands. For God by one understands many things, and that not only according to what they are in themselves, but also according as they are understood, and this is to understand the several types of things. In the same way, an architect is said to understand a house, when he understands the form of the house in matter. But if he understands the form of a house, as devised by himself, from the fact that he understands that he understands it, he thereby understands the type or idea of the house. Now not only does God understand many things by His essence, but He also understands that He understands many things by His essence. And this means that He understands the several types of things; or that many ideas are in His intellect as understood by Him.
Reply to Objection 3. Such relations, whereby ideas are multiplied, are caused not by the things themselves, but by the divine intellect comparing its own essence with these things.
Reply to Objection 4. Relations multiplying ideas do not exist in created things, but in God. Yet they are not real relations, such as those whereby the Persons are distinguished, but relations understood by God.
Source: Summa Theologica (New Advent)