In a recent episode we saw that God’s existence is not self-evident to us. I said at the end of that post that I wanted to take a closer look at what St Thomas has to say about this subject in the Summa Contra Gentiles; that’s what I want to do today. In Book I chapter 10 Aquinas presents arguments that are sometimes offered in defense of the self-evident existence of God. In chapter 11 he answers them so as to show that they are mistaken.
At least some of the arguments offered in defense of the thesis look very much like St Anselm’s ontological argument:
Those things are said to be self-evident which are known as soon as the terms are known: thus as soon as it is known what is a whole, and what is a part, it is known that the whole is greater than its part. Now such is the statement God is. For by this word God we understand a thing a greater than which cannot be thought of: this is what a man conceives in his mind when he hears and understands this word God: so that God must already be at least in his mind. Nor can He be in the mind alone, for that which is both in the mind and in reality is greater than that which is in the mind only. And the very signification of the word shows that nothing is greater than God. Wherefore it follows that it is self-evident that God is, since it is made clear from the very signification of the word.
Again. It is possible to think that there is a thing which cannot be thought not to exist: and such a thing is evidently greater than that which can be thought not to exist. Therefore if God can be thought not to exist, it follows that something can be thought greater than God: and this is contrary to the signification of the term. Therefore it remains that it is self-evident that God is.
Further. Those propositions are most evident in which the selfsame thing is predicated of itself, for instance: Man is man; or wherein the predicate is included in the definition of the subject, for instance: Man is an animal. Now, as we shall show further on, in God alone do we find that His being is His essence, as though the same were the answer to the question, What is He? as to the question, Is He? Accordingly when we say, God is, the predicate is either identified with the subject, or at least is included in the definition of the subject. And thus it will be self-evident that God is.
Moreover. Things that are known naturally are self-evident, for it is not by a process of research that they become evident. Now it is naturally known that God is, since man’s desire tends naturally to God as his last end, as we shall show further on. Therefore it is self-evident that God is.
Again. That whereby all things are known must needs be self-evident. Now such is God. For just as the light of the sun is the principle of all visual perception, so the divine light is the principle of all intellectual knowledge, because it is therein that first and foremost intellectual light is to be found. Therefore it must needs be self-evident that God is.
On account of these and like arguments some are of opinion that it is so self-evident that God is, that it is impossible for the mind to think the contrary.
But these arguments don’t work, he says. In the first place, we often confuse what we hold by long experience with what is genuinely self-evident: these things seem obvious to us because we’ve been accustomed for so long to think a certain way about them. We use a fork and not our hands to eat salad because, well, that’s just what you do. And the same may be said with respect to why many theists believe in God. They were raised to do so; they’ve done so their entire lives; it’s just obvious to them that He exists. Obviously they’re not wrong about the fact that He exists, but that doesn’t mean that it is self-evident.
In the second place he says that these arguments confuse what is self-evident in itself with what is self-evident to us. And although God’s existence may be said to be self-evident in itself, it is by no means self-evident to us:
It is also a result of failing to distinguish between what is self-evident simply, and that which is self-evident to us. For it is simply self-evident that God is, because the selfsame thing which God is, is His existence. But since we are unable to conceive mentally the selfsame thing which is God, that thing remains unknown in regard to us. Thus it is self-evident simply that every whole is greater than its part, but to one who fails to conceive mentally the meaning of a whole, it must needs be unknown. Hence it is that those things which are most evident of all are to the intellect what the sun is to the eye of an owl, as stated in Metaph. ii.
Turning to the arguments presented in chapter 11: The first argument doesn’t work. It claims that as soon as the meaning of God is known the fact that He exists is known. This is not the case.
First, because it is not known to all, even to those who grant that there is a God, that God is that thing than which no greater can be thought of, since many of the ancients asserted that this world is God.
…Secondly because, granted that everyone understands this word God to signify something than which a greater cannot be thought of, it does not follow that something than which a greater cannot be thought of exists in reality. For we must needs allege a thing in the same way as we allege the signification of its name. Now from the fact that we conceive mentally that which the word God is intended to convey, it does not follow that God is otherwise than in the mind. Wherefore neither will it follow that the thing than which a greater cannot be thought of is otherwise than in the mind. And thence it does not follow that there exists in reality something than which a greater cannot be thought of. Hence this is no argument against those who assert that there is no God…
The second argument doesn’t work, because the possibility of thinking of something greater than God doesn’t imply a defect in God but rather in our thinking about Him, which does not attain to His essence at all, and which only knows anything about Him by reasoning from His effects.
The third argument fails for the same reason as the second: we do not know God’s essence at all; we only know Him by means of His effects.
The fourth argument doesn’t work because by nature we don’t desire God Himself, but rather happiness:
For man knows God naturally in the same way as he desires Him naturally. Now man desires Him naturally in so far as he naturally desires happiness, which is a likeness of the divine goodness. Hence it does not follow that God considered in Himself is naturally known to man, but that His likeness is. Wherefore man must needs come by reasoning to know God in the likenesses to Him which he discovers in God’s effects.
The fifth argument doesn’t work, either, because God is not the cause of our knowledge such that we know nothing at all unless we know Him, but rather because He is the cause of our knowledge.
In summary: God’s existence isn’t just obvious. But that doesn’t mean that His existence can’t be demonstrated.
Aquinas, Etc. 
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