1. Having completed his prefatory treatment of the nature and method of theological science, St. Thomas proceeds to outline his intended course of inquiry. First, he will treat of God, then of the rational creature's advance toward God, and finally of Christ, and the path he opens for us to ascend to God. These three points summarize the three parts of the Summa. The Prima Pars discusses God as he exists in himself, and as he is the origin of the existence of creatures. The Secunda Pars discusses the rational creature's striving toward God, and the principles and perfections of rational action, both in general and in particular. The Tertia Pars discusses the incarnation and life of Christ, as well as its soteriological, sacramental, and eschatological implications for Christians.
2. St. Thomas does not stop, however, with an outline of the major parts of the text. Rather, his division of his great task descends further. In the Prima Pars he sees three main areas of inquiry: (1) the Divine Essence, (2) the distinction of the three Persons, (3) the procession of creatures. And, since he is about to embark on the first of these, he graces us with a further sub-division: (1) Whether God exists, (2) The manner of his existence (negative), (3) the divine operations.
3. There is far too much to say about Question 2, and were I try to get through all of it in a systematic way, I would be stuck working on this question for weeks. Instead, I am going to try and be brief and highlight some basic points. St. Thomas begins the treatise on the Divine Essence (commonly called the treatise on the One God) with the question "An Deus sit?" And, since this question terminates in a series of proofs of God's existence, its first article asks whether a proof is necessary--i.e., whether the existence of God is self-evident (note the nested "an est?" -> "quid est?" structure).
4. In favor of the self-evidence of God's existence, St. Thomas proposes three arguments: First, that we call "self-evident" those things knowledge of which is naturally implanted in the human mind. But as Damascene and St. Paul attest, the knowledge of God is naturally open to all. Hence God's existence must be self-evident.
5. Second, that we call "self-evident" those propositions the truth of which is evident simply from the definition of he terms. If we define God as "that being greater than which no being can be conceived", then it is impossible to add the notion of "non-existence" to the notion of "God", since it is recognized that an existing being is superior to a non-existing one, making "God does not exist" a contradiction in terms. Thus we must admit that God exists, and that this truth is self-evident.
6. Third, that since the act of denying the existence of truth is self-contradictory, it is self-evident that truth exists. But God is Truth, and thus to deny the existence of God is to deny the existence of Truth. Therefore it is self-contradictory to say that God does not exist.
7. For the Sed Contra, St. Thomas cites Psalm 52: "The fool hath said in his heart…" But if the fool hath said it, then it must be possible to conceive of the non-existence of God, which means that the proposition in question is not self-evident. Truly self-contradictory judgments cannot be entertained by the mind.
8. In order to resolve the problem of this article, St. Thomas distinguishes between two modes of self-evidence. Some things are self-evident in themselves, and of these a subset is self-evident to us. For a proposition to be self evident "to us", the essence of the thing spoken of must necessarily imply the predicate attributed to it through the available content of the concepts involved.
9. An illustration: Suppose we are in Egypt, a few thousand years ago, and suppose that we are part of a medical community which sees the brain and nervous system as a bunch of fibrous condensed mucus deposited by the body to fill its extra spaces. (Bracket the question of whether anyone believed this.) In this case we might reasonably suppose that it is possible for there to be human beings without nervous systems, and this would not be self-contradictory. However, given an understanding of human nature in which core intellectual, perceptual, and motor capacities were tied up with the existence and health of the nervous system, to the extent that without a nervous system, the organic basis of the life and unity of the human body would be absent, then it would be impossible to say that human beings can exist without nervous systems, because the notion of "humanity" would be so deeply tied to the activity of that set of organs.
10. In one situation, the judgment is self-evident. In another it is not self-evident. But note that where the judgment is self-evident, it is such because we have arrived at a different understanding of the essence of the thing under consideration—i.e., in this case we have become able to more accurately define "humanity." Hence, apart from this or that specific self-evidence, which is dependent on the particular notions employed in our understanding of what a thing is, there is the question of real implication, apart from any understanding or judgment. Assuming that our definition of humanity is correct, there is an intrinsic self-evidence to the judgment that humans have nervous systems, apart from any particular perspective or understanding of the matter. This is self-evidence secundum se, according to the thing. The availability of this self-evidence to a particular individual based on that individual's grasp of the essence of a thing is called self-evidence quoad nos, to us.
11. Now we apply this distinction to the question of God's existence. That God's existence is self-evident secundum se will be shown later on, when we deal with Divine Simplicity. However, for the moment it is quite evident that God's existence cannot really be self-evident quoad nos. This is because every definition we give of God is as inadequate as the supposed Egyptian understanding of humanity, and more so. God's essence, being utterly simple, perfect, infinite, and immaterial, we cannot comprehend it, and therefore cannot properly define it. Furthermore, in the natural order of human knowledge we can know the divine nature only indirectly, through creatures. One could as little describe the haircolor of a man by looking at his shadow as attempt to define the essence of God by inferences from the created world.
12. But how do we deal with the initial objections? To the first, we must reply that while Saints Paul and John Damascene are correct in saying that God's nature is, to some extent, evident to all, this evidence comes through our experience of the created world. It does not extend to self-evidence, which would require proper knowledge of God as he is in himself.
13. To the second, St. Thomas famously rejects Anselm's argument as begging the question. It is true that any being greater than which none can be conceived would have to exist, but whether there is such a being is left undetermined by that line of thought. The addition of "existence" to the criteria collected in a mental concept does not immediately allow us to declare that the concept is instantiated in reality. People often claim that Anselm's actual argument is subtle (and perhaps valid) in a way that Aquinas misses. I have yet to hear a presentation of the argument, though, that isn't (despite some semantic contortionism) reducible to the one Aquinas demolishes.
13. To the third, St. Thomas answers simply that while the existence of truth in general is self-evident, the existence of "First Truth" (i.e., God) is not self-evident. The argument proposed only demonstrates the existence of the former.
Outline of Article
–St. Paul and St. John Damascene both attest to the natural availability of knowledge of God
–God is that being greater than which none can be conceived, and therefore must exist by definition.
–To say that truth does not exist is a contradiction, but God is Truth.
–The fool hath said in his heart, 'there is no God.'
–There are two varieties of self-evidence. One relative and subjective, the other objective, in the thing itself.
–The existence of God is self-edvident in itself, because it is essential to the divine nature that it exists.
–However, since we cannot know God's nature in this life, except indirectly, we cannot apprehend the self-evidence of this truth.
–The knowledge of God is evident through creatures, but not self-evident.
–This argument still leaves the existence of such a being undetermined.
–Truth exists in common, as the argument attests, but this does not demonstrate the existence of a first truth, which is God.