2.2 Faith versus Demonstration (SCG 1,12)
2.2 Faith versus Demonstration (SCG 1,12)
So Aquinas denies that “God exists” is self-evident to us, and this conclusion will lead him in the SCG to consider how God’s existence might be demonstrated. Yet now we come to the objection to natural theology grounded on faith. Should we not hold that “God exists” cannot be demonstrated since it is held to be true, or should only be held to be true, on the basis of divine revelation? This is the question that Aquinas is concerned with in SCG 1,12, and his answer to it is no.
As I noted in chapter 1, Aquinas has no problem with the notion of divine revelation. Indeed, to suggest that he does would be grossly to misrepresent him. He firmly believes that God has revealed truths that we cannot demonstrate to be true. He also holds that someone might be warranted in believing that God exists just by reading the Bible and by taking it to contain divine revelation. Yet Aquinas is also aware of arguments suggesting that the project of demonstrating that God exists is doomed on philosophical grounds. As I have said, some theologians have held that natural theology is an abomination to be theologically condemned. But Aquinas does not refer to any such theologians in SCG 1,12. Rather, he starts by vaguely alluding to the claim that purported demonstrations that God exists fail as demonstrations because they amount to weak arguments. He then proceeds to note precise objections to the quest for demonstrations of the truth of “God exists,” objections that he clearly takes to be philosophical ones.10
They can be presented thus:
1. If God’s essence is to be, if God’s essence and existence are identical, and if we cannot know what God is, then we can hardly demonstrate that God exists.11
2. Aristotle says that in demonstrating whether something exists we must begin with an understanding of what we name by the thing in question.12 He also says that the meaning signified by a name is its definition.13 But we cannot rely on an understanding of God when it comes to demonstrating that God exists if we do not have a knowledge of God’s essence or “whatness” (quidditas).
3. Again, Aristotle says that we know how to provide demonstrations insofar as we are dealing with objects open to sensory investigation.14 But God is no such object.
Yet Aquinas denies that these arguments show that “God exists” is not demonstrable. Or, rather, he denies that they show that “God exists” is in no sense demonstrable since there are two kinds of demonstration. “In arguments proving the existence of God it is not necessary to assume the divine essence or quiddity as the middle term of the demonstration. … In place of quiddity, an effect is taken as the middle term, as in demonstrations quia.”15
Following Aristotle, Aquinas takes a demonstration to be an argument with evidently true premises that entail its conclusion. More precisely, he takes a demonstrative argument basically, though not always exactly, to have the form: (1) “All X is Y” (e.g., “All human beings are mammals”); (2) “All Y is Z” (e.g., “All mammals breath air”); (3) “Therefore, all X is Z” (e.g., “Therefore, all human beings breath air”). With that said, however, I also need to note that Aquinas recognizes two kinds of causal demonstration: (1) from cause to effect, and (2) from effect to cause.
In Summa Theologiae 1a,2,2 he writes: “There are two kinds of demonstration. One kind, propter quid (‘on account of which’ or ‘of the reason for’), argues from cause to effect and proceeds by means of what is unqualifiedly first. The other, demonstration quia (‘that’), argues from effect to cause and proceeds by means of what is first so far as we are concerned.”16 What is Aquinas thinking here? His distinction can be grasped by means of examples.
Suppose we reason thus:
1. Hydrogen is the element with atomic number one.
2. The element with atomic number one is the lightest gas.
3. So hydrogen is the lightest gas.
4. Any balloon filled with the lightest gas rises in air.
5. So any balloon filled with hydrogen rises in air.
Here we start from a definition of what something is, and we end up arguing syllogistically for the occurrence of a visible effect. Our procedure here is an example of what Aquinas has in mind by “demonstration propter quid,” which he thinks of as an argument from cause to effect starting with a grasp of the nature of the cause.
However, suppose we reason:
1. This balloon is rising in the air.
2. Every rising balloon is full of something that makes it rise.
3. So this balloon is full of something that makes it rise.
4. Everything that fills a balloon and makes it rise must be lighter than air.
5. So this balloon is full of something lighter than air.
Here we make no appeal to what it is precisely that fills this balloon rising in the air; we do not claim to know what exactly that is. Is it hydrogen, helium, or just hot air? But we do try to account for what we perceive in terms of a cause of some sort. We reason from effect to cause. Our procedure here is an example of what Aquinas has in mind when he refers to demonstration quia, which he thinks of as a syllogistic argument from effect to cause.17
Now, according to Aquinas, if we know, and do not just believe, that “God exists” is true, that can only be by virtue of a demonstration quia.18 Demonstration propter quid is, he thinks, impossible when it comes to the truth of “God exists.” Why? Because, as I have now shown, Aquinas thinks that we do not understand what God is, because we do not know God’s nature or essence. The second objection in Summa Theologiae 1a,2,2 runs: “The middle term in a demonstration is what something is. But, as Damascene tells us, we do not know what God is, only what he is not.”19 The objection in question here is arguing that one cannot at all demonstrate that God exists, and Aquinas denies that conclusion. However, he does not quarrel with what I have just quoted from the objection. Instead, he draws attention to the fact that there are two kinds of demonstration, one of which does not presume a knowledge of what God is. Then he says that one can demonstrate that God exists by means of an argument from effect to cause. “When we demonstrate a cause from its effect,” he observes,
the effect takes the place of what the cause is in the proof that the cause exists, especially if the cause is God. For, when proving that something exists, the middle term is not what the thing is (we cannot even ask what it is until we know that it exists) but what we are using the name of the thing to mean. But when demonstrating from effects that God exists, we are able to start from what the word “God” means.20
This is the line that Aquinas takes in SCG 1,12. In his view, there can be demonstration quia for the truth of “God exists,” even if we cannot understand what God’s essence is. Such demonstration will not offend against the idea that demonstration “takes its origin from sense” since it will amount to concluding to “God exists” on the basis of sensible things. “Although God transcends all sensible things and the sense itself, God’s effects, on which the demonstration proving his existence is based, are nevertheless sensible things.”21 Aquinas takes this conclusion to cohere with what St. Paul says in Romans 1:20: “Ever since the creation of the world his [i.e., God’s] eternal power and divine nature, invisible though they are, have been understood and seen through the things he has made.”22 Indeed, says Aquinas, even Aristotle thought that there could be demonstration concerning what is not a “sensible substance,” something we can know about by means of our senses.23