In The Cosmological Argument from Plato to Leibniz (1980) William Lane Craig writes this about Aquinas’s five arguments for God’s existence in the Summa:
Thomas’s proofs for the existence of God encompass the whole of Ia. 2-11 of the Summa Theologiae. Modern readers, used as they are to anthologized versions of Aquinas’s Five Ways, all too often fail to grasp this important point. Aquinas is sometimes criticized for what is thought to be his over-hasty conclusion: ‘…and this is what everybody understands by God’; but this misunderstanding only arises by tearing Aquinas’s proofs out of their proper context.
I think Craig is right that the God whose existence Aquinas is proving in the Summa can only be properly understood if we read the questions that follow q. 2, a. 3. So, neither can the Five Ways be properly understood apart from these questions. (But this does not mean — as Craig also acknowledges — that on their own the Five Ways do not prove that God exists under certain limited descriptions.)
Craig’s insight is not original, nor does he claim it to be. Many Aquinas scholars grasp the point and for some it is a commonplace. Gerard Smith, one of last century’s more prominent American (and Jesuit) Thomists, in his Natural Theology (1951), which largely restates the relevant parts of the Summa, writes:
After the proof of God’s existence there remains only the investigation of His names or nature. The proof of God’s existence and the investigation of His nature are not exactly related as old to new and different knowledge. Rather, the two are related as the less well to the better known. Once God has been proved to exist as a pure act of existing, anything further to be said concerning God is merely a long commentary, as some have remarked, upon the proof. The only reason, therefore, for separating the two treatises is merely one of order: one cannot say everything about God in the same breath; yet one must have known, so to say, in one breath, the nature of God if one has seen the proof of Him. To have seen that God’s nature is to exist is to see in germ whatever else can be said of Him.
It is hard to imagine anyone putting it better.
There is another comment that should be made about the quote from Craig. There is a point on which he is certainly wrong. As we saw, Craig says that “Thomas’s proofs for the existence of God encompass the whole of Ia. 2-11 of the Summa Theologiae.” He is right, as I have noted, that the questions that follow q. 2, a. 3 must be taken into account to understand the God Aquinas talks about in the Five Ways properly. But Craig is mistaken to limit the post-q. 2 questions to qq. 3-11. Craig further stresses his point by affirming that “[i]t is not until the finish of question 11 that the existence of what we mean by ‘God’ has been demonstrated.”
This may be true for Craig but it is not true for Aquinas and so Craig’s interpretation is flawed. Why do I say this? Here is what qq. 3-11 treat:
q. 3 divine simplicity
q. 4 divine perfection
q. 5 goodness in general
q. 6 divine goodness
q. 7 divine infinity
q. 8 divine omnipresence
q. 9 divine immutability
q. 10 divine eternity
q. 11 divine unity
Immediately after, in q. 12 Aquinas deals with our knowledge of God and in q. 13 he considers how God is named by us (i.e., how language can refer to God). But Aquinas has not stopped talking about God himself. In qq. 14-18 he discusses God’s intellect, in qq. 19-21 God’s will, in q. 22-24 God’s intellect and will together, in q. 25 God’s power, and in q. 26 God’s beatitude. In other words, to understand rightly the God whose existence Aquinas argues for in q. 2, a. 3 we cannot stop at q. 11, as Craig suggests; there is reason to think that qq. 12-13 should be included and it is plain that qq. 14-26 should be included. We should probably also add qq. 44-46, which treat of God the Creator.
In qq. 27-43 Aquinas discusses God as he is known through revelation, i.e., the trinitarian God. But Aquinas denies that the Trinity can be demonstrated by reason. So, these questions evidently are not, for Aquinas, included in the demonstration of God’s existence adequately considered. Nevertheless, in Aquinas’s view, the God who is demonstrated by reason is numerically identical with the God who supernaturally reveals himself. But he is not formally identical.
These remarks bring us to one further problem I would like to point out with Craig’s reading of Aquinas. If we are talking about Aquinas, “what we mean by ‘God’” is not only the God knowable by reason, which is the God that Craig seems to have in mind in that phrase. For Aquinas, “what we mean by ‘God’” is the unus Deus, the one God, who we can know partly by reason and fully by faith.
Everything that I have said here seems so obvious as to be uninteresting, so I assume that Craig would not disagree and that the problems that I mention in his comments on Aquinas stem from a momentary slip that he would happily correct were it indicated.