Objection 1. It would seem that among the divine persons there does not exist an order of nature. For whatever exists in God is the essence, or a person, or a notion. But the order of nature does not signify the essence, nor any of the persons, or notions. Therefore there is no order of nature in God.
Objection 2. Further, wherever order of nature exists, there one comes before another, at least, according to nature and intellect. But in the divine persons there exists neither priority nor posteriority, as declared by Athanasius. Therefore, in the divine persons there is no order of nature.
Objection 3. Further, wherever order exists, distinction also exists. But there is no distinction in the divine nature. Therefore it is not subject to order; and order of nature does not exist in it.
Objection 4. Further, the divine nature is the divine essence. But there is no order of essence in God. Therefore neither is there of nature.
On the contrary, Where plurality exists without order, confusion exists. But in the divine persons there is no confusion, as Athanasius says. Therefore in God order exists.
I answer that, Order always has reference to some principle. Wherefore since there are many kinds of principle--namely, according to site, as a point; according to intellect, as the principle of demonstration; and according to each individual cause--so are there many kinds of order. Now principle, according to origin, without priority, exists in God as we have stated (33, 1): so there must likewise be order according to origin, without priority; and this is called 'the order of nature': in the words of Augustine (Contra Maxim. iv): "Not whereby one is prior to another, but whereby one is from another."
Reply to Objection 1. The order of nature signifies the notion of origin in general, not a special kind of origin.
Reply to Objection 2. In things created, even when what is derived from a principle is co-equal in duration with its principle, the principle still comes first in the order of nature and reason, if formally considered as principle. If, however, we consider the relations of cause and effect, or of the principle and the thing proceeding therefrom, it is clear that the things so related are simultaneous in the order of nature and reason, inasmuch as the one enters the definition of the other. But in God the relations themselves are the persons subsisting in one nature. So, neither on the part of the nature, nor on the part the relations, can one person be prior to another, not even in the order of nature and reason.
Reply to Objection 3. The order of nature means not the ordering of nature itself, but the existence of order in the divine Persons according to natural origin.
Reply to Objection 4. Nature in a certain way implies the idea of a principle, but essence does not; and so the order of origin is more correctly called the order of nature than the order of essence.
Source: Summa Theologica (New Advent)