Objection 1. It would seem that the exemplar cause is something besides God. For the effect is like its exemplar cause. But creatures are far from being like God. Therefore God is not their exemplar cause.
Objection 2. Further, whatever is by participation is reduced to something self-existing, as a thing ignited is reduced to fire, as stated above (Article 1). But whatever exists in sensible things exists only by participation of some species. This appears from the fact that in all sensible species is found not only what belongs to the species, but also individuating principles added to the principles of the species. Therefore it is necessary to admit self-existing species, as for instance, a "per se" man, and a "per se" horse, and the like, which are called the exemplars. Therefore exemplar causes exist besides God.
Objection 3. Further, sciences and definitions are concerned with species themselves, but not as these are in particular things, because there is no science or definition of particular things. Therefore there are some beings, which are beings or species not existing in singular things, and these are called exemplars. Therefore the same conclusion follows as above.
Objection 4. Further, this likewise appears from Dionysius, who says (Div. Nom. v) that self-subsisting being is before self-subsisting life, and before self-subsisting wisdom.
On the contrary, The exemplar is the same as the idea. But ideas, according to Augustine (QQ. 83, qu. 46), are "the master forms, which are contained in the divine intelligence." Therefore the exemplars of things are not outside God.
I answer that, God is the first exemplar cause of all things. In proof whereof we must consider that if for the production of anything an exemplar is necessary, it is in order that the effect may receive a determinate form. For an artificer produces a determinate form in matter by reason of the exemplar before him, whether it is the exemplar beheld externally, or the exemplar interiorily conceived in the mind. Now it is manifest that things made by nature receive determinate forms. This determination of forms must be reduced to the divine wisdom as its first principle, for divine wisdom devised the order of the universe, which order consists in the variety of things. And therefore we must say that in the divine wisdom are the types of all things, which types we have called ideas--i.e. exemplar forms existing in the divine mind (15, 1). And these ideas, though multiplied by their relations to things, in reality are not apart from the divine essence, according as the likeness to that essence can be shared diversely by different things. In this manner therefore God Himself is the first exemplar of all things. Moreover, in things created one may be called the exemplar of another by the reason of its likeness thereto, either in species, or by the analogy of some kind of imitation.
Reply to Objection 1. Although creatures do not attain to a natural likeness to God according to similitude of species, as a man begotten is like to the man begetting, still they do attain to likeness to Him, forasmuch as they represent the divine idea, as a material house is like to the house in the architect's mind.
Reply to Objection 2. It is of a man's nature to be in matter, and so a man without matter is impossible. Therefore although this particular man is a man by participation of the species, he cannot be reduced to anything self-existing in the same species, but to a superior species, such as separate substances. The same applies to other sensible things.
Reply to Objection 3. Although every science and definition is concerned only with beings, still it is not necessary that a thing should have the same mode in reality as the thought of it has in our understanding. For we abstract universal ideas by force of the active intellect from the particular conditions; but it is not necessary that the universals should exist outside the particulars in order to be their exemplars.
Reply to Objection 4. As Dionysius says (Div. Nom. iv), by "self-existing life and self-existing wisdom" he sometimes denotes God Himself, sometimes the powers given to things themselves; but not any self-subsisting things, as the ancients asserted.
Source: Summa Theologica (New Advent)