1. Having demonstrated that the existence of God is not self-evident to us, St. Thomas next asks whether it is possible to demonstrate that God exists at all. If the previous article is a safeguard against overconfidence in the power of natural human reasoning, then this article is a safeguard against a particular sort of pessimism about the powers of human reason. In our present context the second is probably more interesting and certainly more useful.
2. Against the demonstrability of God's existence, Aquinas proposes three arguments: First, that the existence of God is an article of faith, and since faith is a supernatural knowledge of things "unseen", as Hebrews says (11:1), it would be improper to presume the natural knowability of this fact.
3. Second, he points out a feature of ordinary arguments. In order to establish that a thing of a certain species has a certain quality, one must know the essence of the species. "Socrates is a man, and all men are mortal, therefore Socrates is mortal." Here the essence of Socrates, i.e. the sort of thing that he is ("man"), forms the middle term which enables us to connect "Socrates" to the predicate "mortal". But if we are attempting to show that the predicate "exists" belongs to the subject "God", then it seems we must be able to know that existence is a feature of the divine essence, i.e., of that sort of thing that God is, and thus we must know God's essence itself. This is, however, impossible, as St. John Damascene says in Book I, Chapter IV of his De Fide Orthodoxa.
4. Third, Thomas observes that God can be known only through his effects (since he is invisible and immaterial), but that since God is an infinite being, whereas everything not God is actually finite (though it may be potentially infinite, as we will discuss in question 8), and one cannot deduce from a finite effect the existence of an infinite cause, it follows that the existence of God cannot be demonstrated from creatures, and thus cannot be demonstrated at all.
5. Of these three arguments, variants of the first are found most frequently on the lips of protestants, for whom in many cases the claims of natural theology threaten to set up a rationalistic idol in the place of the super-transcendent deity, who is revealed to us primarily in the cross (thus Luther's "theologia crucis").
6. Aquinas replies with a citation from the first chapter of Romans, which is a key source for traditional confidence in the natural knowability of God apart from revelation. (N.B. it is an article of faith that God's existence can be known apart from faith.) In Latin, the text reads "invisibilia Dei per ea quae facta sunt, intellecta, conspiciuntur". Translation: "the invisible things of God, being understood through the things which are made, are clearly observed." The Greek text, τὰ γὰρ ἀόρατα αὐτοῦ ἀπὸ κτίσεως κόσμου τοῖς ποιήμασιν νοούμενα καθορᾶται, translates as "for the invisible things of him are understood by the things that have been made/done, from the foundation of the world, which are observed." The Greek text is significantly stronger than the Vulgate in making the point Aquinas wants. In either case, clearly one major implication of the text is that God can be known through creatures, and if he can be known, then he must be known to exist.
7. Before we turn to the corpus, let it be noted that the latin phrases "a priori" and "a posteriori" used in the standard translation of this article are additions of the translators not found in the original. Furthermore, because the phrases have, since the Enlightenment (and especially Kant) taken on a very particular sense that is more or less alien to St. Thomas's thought, the resulting translation is extremely misleading. Reading the English text, one would think that Thomas is distinguishing between empirical and non-empirical demonstrations in a Kantian fashion.
8. Here is a corrected translation of the corpus: "I respond that it is to be said that demonstration is twofold. One of which is through the cause, and is called "propter quid" (lit. "on account of that"). The other is through the effect, and is called "demonstration quia" (lit. "because"), and this is through things which are prior to us, for when the effect of something is more manifest to us than its cause, we proceed to knowledge of the cause. And from any effect whatsoever it is possible to demonstrate that its proper cause exists (as long as its effects are better known to us), because, when the effects depend on a cause, it is necessary to posit the preexistence of the cause. Whence that God exists, according as it is not known to us through itself (per se notum quoad nos), is demonstrable through the effects known to us."
9. The reader may more easily grasp the difference between demonstrations "propter quid" and "quia" if I give examples of them, using the respective name in each. (The actual genealogy of the terms goes back to Aristotle's Posterior Analytics.) Example of "propter quid" demonstration: The sun is composed mainly of extremely dense plasma undergoing nuclear fusion, and ON ACCOUNT OF THAT it must emit a huge number of neutrinos. Here our inference to the existence of the effect proceeds from a better known fact about the essence of the cause (i.e., what the sun is and how it acts). "X, and on account of that, Y." Example of demonstration "quia": The rotational velocity of galaxies observed to contain a certain quantity of luminous matter must be so great BECAUSE of a large quantity unobserved ("dark") matter in those galaxies. Here our inference to the existence and characteristics of a cause proceeds from a better known effect. "X, because Y."
10. Note that the use of either type of demonstration depends on the particular knowledge available to the individual doing the demonstration. To someone who knew about dark matter first, a propter quid demonstration could be made to its effects on galactic rotation. To someone who observed solar neutrinos first, a demonstration quia could be made to the existence of fusion reactions in the sun.
11. As for the argument for the demonstrability of God's existence, St. Thomas is very modest in his claim here, as will be seen in his responses to the objections. He has shown that the existence of a cause of certain effects is demonstrable. He has not specified to what extent the divine attributes are knowable through created effects, just that it can be known that something (which happens to be identical with what we call "God") exists. The modesty of his claim here will prove important through the rest of this question. The treatise on the One God (Ia qq.2-26) will be mainly occupied with showing the extent to which it can be known that this something (which we call God) has the properties ordinarily ascribed to the being we call God on the basis of faith. (Ia qq. 3-4 are, therefore, in a way just as crucial to the demonstration of God's existence as q.2.) Hence as we move on St. Thomas will make increasing use of divine revelation, both for the sake of comparison to this naturally available knowledge, and as a principle of demonstration in its own right. The Summa Theologiae is a Summa of Sacra Doctrina, remember, and not a work of apologetics. Thus if the distinction between what is naturally known concerning the attributes of God and what is known through revelation is not clearly maintained (which, it seems, it is not), we can infer that this is because the difference is not hugely important to Aquinas, who presumes faith in his students. The Summa Contra Gentiles, however, does not presume faith, and thus maintains the distinction between natural knowledge and the data of revelation throughout.
12. Now we turn to the replies to the objections. In the first reply, St. Thomas unleashes a series of theological principles central to much of his thought. First, he distinguishes between articles of faith and "preambles to the articles." The extent of the so-called "Praeambula Fidei" has been much discussed, particularly because of the ambiguity mentioned above between the use of revelation and that of natural reason in the early questions of the Summa.
13. Second, he says that faith presupposes natural knowledge, just as grace presupposes nature, and perfection something to be perfected. The implication is that without some sort of natural knowledge, one cannot have faith, i.e., that in faith natural acts of knowing are elevated and perfected by grace. This will become much more important later on (esp. in IIa IIae).
14. Nevertheless, St. Thomas informs us, nothing prevents someone from accepting on the basis of faith something that he might otherwise know by natural means. In other words, he is claiming that there are truths which can be believed on the basis of faith which are nonetheless available to all, even apart from grace. It seems to me that this reply shows the fluidity of Thomas's conception of an "article of faith". As we will see (for example, in IIa IIae q.1 a.8 ad 1), Thomas believes the articles of faith are enumerable, but it seems here that the idea of an "article" is simply "something held on the basis of supernatural faith and received directly from general revelation." In this way the articles of faith, though enumerable, are not exhaustively or definitively so.
15. To the second objection, St. Thomas replies that what is needed in a demonstration is not definitive knowledge of the essence of the subject, but simply a knowledge of the meaning of the name given in place of that essence. If this were not the case, it would be impossible to prove the existence of anything, since direct knowledge of a thing's essence is only possible on the basis of observation. Were one to demonstrate, for example, that unicorns exist, it could not be demanded that one specify the essence of a unicorn, but merely the meaning of the name.
16. Consider the following demonstration: "A kind of equine animal has been observed which has a horn growing out of its forehead. But this is what everyone understands by the name 'unicorn'. Therefore unicorns exist." Supposing for a moment the truth of the minor premise (such an animal has been observed), this argument does prove the existence of unicorns, despite the fact that it has invoked not the essence of the creature in question, but only the meaning of a common name. This principle of using the definition of a term as the major premise in a syllogism will be crucial to the next article, as it is employed in every single one of Aquinas's "Five Ways". In any case, here it suffices to say that God's name is derived from his effects (in the first instance, "the cause of such and such"), which is enough to ground a demonstration of the existence of such a thing. (The naming of God from his effects will be discussed in detail in Ia q.13.)
17. To the third objection, Aquinas answers (as should be expected from what we have already said) that though a demonstration "quia" may not provide one with knowledge of a cause fully proportionate to its actuality, i.e. may not provide us with direct knowledge of its essence, still, it can provide us with knowledge of its existence. In this case the knowledge of God's existence will not match up with the fullness of his nature, i.e., from the principles naturally available to us we cannot know the divine nature perfectly, but we can know that such a being as corresponds to the common meaning of the name "God" exists.
Outline of Article:
—God's existence is an article of faith.
—In order to demonstrate that a member of a species has a certain attribute, one must know the essence of the species, which is impossible in the case of God.
—God can only be known through his effects, which are disproportionate to his nature.
—St. Paul says God is known through his visible effects. If he is known, then he must be known to exist.
—There are two kinds of demonstration:
—"propter quid" demonstrations proceed from a better known cause to a less known effect
—"quia" demonstrations proceed from a better known effect to a less known cause
—The existence of God can be demonstrated from his created effects.
—The existence of God can be held on the basis of faith, though it is knowable through reason.
—The definition of a name (derived from its effects or commonly specified attributes) suffices in a demonstration of a thing's existence, since otherwise no such demonstrations would be possible.
—Though a demonstration "quia" of God's existence cannot prove that the thing we call "God" has all of the actual attributes of God, still it can show that such a thing as corresponds to the common meaning of the name "God" exists.