In this article I will attempt to compare the thought of Thomas Aquinas and David Hume concerning the existence of God. I will focus primarily on Thomas' attempt to prove the existence of God, and what Hume may have said in response to it.
One thing that stands out very clearly is their common gift of intellectual prowess. These two are among the most influential minds in Western civilization. Even though their conclusions are very different, there is much truth to be gleaned from both of them.
The major divergence in their thought would, of course, be in religious matters. Thomas was very much a believer in the God of Christianity. Hume was, according to some, an atheist. His writings reveal that he was probably an agnostic. He was, however, very interested in religion and wrote much on the subject.
Aquinas attempted to prove the existence of God in his classic work, Summa Theologica. He presented five very persuasive arguments which he called the "Five Ways." A primary motif of his arguments was God as "First Cause."
A few of the arguments depend heavily upon cause and effect, and a connection between them. His position on demonstrating the existence of God rests on this statement:
...from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because, since every effect depends upon its cause, if the effect exists, the cause must preexist (Summa Theologica, Part I, Question Two, Second Article; emphasis mine).Thomas seems to be saying in this passage that there is a necessary connection between cause and effect because of the dependence of the effect upon a preexistent cause. I understand that the causal chain for Thomas is not an infinite series, but is rather a hierarchy of causal activity in which a subordinate cause is dependent upon a higher cause. Nevertheless, I still think he is claiming a necessary connection.
This is the very thing that Hume attacked so vehemently in his philosophical writings. He tells us that a necessary connection cannot be observed. In his Enquiry Concerning Human Understanding, he says,
When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connection; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other" (Section VII).If this be true, the arguments presented from causality by Aquinas are highly questionable. Since there is no necessary nexus between cause and effect, there can be no chain leading back to a First Cause.
I don't think Hume would have had a problem with Aquinas saying he "believed" there was some connection between cause and effect. Hume taught that "belief" is a strong feeling "which distinguishes the ideas of the judgment from the fictions of the imagination" (ibid.). Belief, in this case, is based on past experience, or "custom," as Hume calls it. We "believe" that when a billiard ball strikes another billiard ball the latter will move. We do so because it has always happened like that in the past. But Aquinas is not stating a belief in this manner. He has taken for granted that a necessary connection exists between cause and effect, and
is making a logical inference, from effects, that a First Cause must exist. He has done this without establishing first that a necessary connection truly exists.
But what would Thomas say to Hume in response to the problem of a necessary connection? He might agree with Hume that one cannot observe a necessary connection. He might say that humans are still aware of a causal relation, nevertheless.
F.C. Copleston discusses this in his book, "Aquinas." He says:
A remark on the word 'cause' is here in place. What precisely Aquinas would have said to the David Humes either of the fourteenth century or of the modern era it is obviously impossible to say. But it is clear that he believes in real causal efficacy and real causal relations. He was aware, of course, that causal efficacy is not the object of vision in the sense in which patches of colours are objects of vision; but the human being, he considered, is aware of real causal relations and if we understand 'perception' as involving the cooperation of sense and intellect, we can be said to 'perceive' causality (page 123).When Copleston says Thomas was aware that "causal efficacy is not the object....," he seems to be saying that Aquinas was aware that a necessary connection could not be observed, but that we could, nevertheless, "perceive" such a connection.
Hume, in Part IX of the Dialogues Concerning Natural Religion, goes on to further criticize the idea of a First Cause. He says the idea "seems absurd," and that the act of uniting causes and effects into a whole, which may seem to demand a cause, is merely an "arbitrary act of the mind, and has no influence on the nature of things."
Aquinas' third way asserts that God is a necessarily existing Being. He arrives at this conclusion by asserting that God cannot not-be, therefore He exists necessarily.
Hume, in his Dialogues Concerning Natural Religion, answers this argument through the mouth of Cleanthes:
Nothing is demonstrable unless the contrary implies a contradiction. Nothing that is distinctly conceivable implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no being, therefore, whose non-existence implies a contradiction. Consequently, there is no being whose existence is demonstrable (Part IX).Since the mind can conceive of God as not existing, His/Her existence cannot be demonstrated.
Thomas would probably argue along the lines of necessity. I think he would say that Hume's argument would apply only to beings that come into existence or pass away. Hume may not have understood the way theologians like Aquinas and Anselm used the term "necessary being." Aquinas did not mean that God was a logically necessary being, but that God's necessity is factual, or equivalent to the self-existence (aseity) of God. God's necessary being should not be thought of as equivalent to saying the proposition "God exists" is a logically necessary truth.
It seems that both men have interesting things to say. As philosophers, we can learn much from their writings. We need not be religious to learn from Thomas, and we need not be a skeptic to glean from Hume. They both exhibit astounding intellects, and have both helped to shape the course of our present world.