In the first question of the Summa Theologiae St Thomas addresses some preliminary issues relating to the value, nature, and extent of sacred doctrine (the science of divine revelation). With the second question he begins to treat of theology proper (i.e., the doctrine of God Himself), and he opens the discussion thus:
Because the chief aim of sacred doctrine is to teach the knowledge of God, not only as He is in Himself, but also as He is the beginning of things and their last end, and especially of rational creatures, as is clear from what has already been said, we shall treat: (1) Of God; (2) Of the rational creature’s advance towards God; (3) Of Christ, Who as man, is our way to God. [p. 11 of volume 1 of this edition; this portion of the Summa is omitted from New Advent’s online version]
There is something of a plan here for what he intends to cover in the rest of the work, which you can see spelled out a bit more here.
He begins the discussion of the doctrine of God with the subject of His existence. Knowing that a thing exists is the first step towards knowing anything else about it, and this is true with God as with anything else. But before he gets to the proofs for God’s existence he addresses two mistakes related to whether His existence can be demonstrated or not. The first error is to think that God’s existence does not need to be demonstrated at all because His existence is self-evident. Before we go too far, though, we need to know what St Thomas means by “self-evident.”
A proposition is self-evident because the predicate is included in the essence of the subject, as “Man is an animal,” for animal is contained in the essence of man. If, therefore the essence of the predicate and subject be known to all, the proposition will be self-evident to all; as is clear with regard to the first principles of demonstration, the terms of which are common things that no one is ignorant of, such as being and non-being, whole and part, and such like. [ST Ia q2 a1]
We know that a whole is greater than any of its parts just because of what “whole” and “part” mean. We know that “man is an animal” because “animal” is part of the definition of “man” (which in this context is “rational animal”). We know that 2 is greater than 1. And so forth. These are self-evident statements, and Aquinas says that God’s existence does not qualify as self-evident.
If, however, there are some to whom the essence of the predicate and subject is unknown, the proposition will be self-evident in itself, but not to those who do not know the meaning of the predicate and subject of the proposition. Therefore, it happens, as Boethius says (Hebdom., the title of which is: “Whether all that is, is good”), “that there are some mental concepts self-evident only to the learned, as that incorporeal substances are not in space.” Therefore I say that this proposition, “God exists,” of itself is self-evident, for the predicate is the same as the subject, because God is His own existence as will be hereafter shown (3, 4). Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature — namely, by effects. [ibid.]
Some things are self-evident to those who have sufficient knowledge of them but not to others; this is not a defect of the self-evident thing itself, but rather a defect of our knowledge. And because (as Aquinas says) we don’t know God’s essence, it would be impossible for a proposition about Him to be self-evident. Instead, truths about God must be made known to us. They need to be shown either by demonstration (where that is possible) or by revelation. God’s existence is like that: it’s not self-evident, and so it must be demonstrated (as he intends to do shortly).
It is sometimes said (and more often it is forgotten) that the Summa Theologiae does not propose to offer comprehensive argumentation for every point that it makes. Given the sheer size of the work and the depth of detail that it does present it’s very easy to forget this! But the book is a textbook. It doesn’t cover all the details. In the present case we see this if we take a look at chapters 10 and 11 of book I of the Summa Contra Gentiles, where St Thomas presents a more complete argument against the idea that God’s existence is self-evident. But this post is long enough, so we’ll save that for another day (see here).