The existence of God can be rationally proven, and Thomas Aquinas does a better job of this than Anselm. Aquinas uses strictly logical reasoning while Anselm starts off with a biased attitude; his opinion is clearly shown by his quotation from the Bible: “unless I believe, I shall not understand,” (Isa. 7:9). No such traces are evident in the works of Aquinas, whether in an obvious form such as a religious excerpt or in his argument’s logic itself. If only because of this, one is able to take Aquinas’ assertions more seriously than Anselm’s, because there is no sense of proclivity toward Christianity in his work.
Beyond the obvious leaning toward Christianity that Anselm has, his arguments make less sense because he claims things to be true without giving reasons why. “But clearly that than which a greater cannot be thought cannot exist in the understanding alone. For if it is actually in the understanding alone, it can be thought of as existing also in reality, and this is greater,” (Ans., Prosl.). In this excerpt, Anselm is trying to say that God, the entity than which a greater cannot be thought, must exist because reality is greater than understanding, and so if we can think of God, God exists. That is because if we can think of It as existing in reality, God would be less than if It actually did exist in reality, and so God must therefore exist in reality in order to be the greatest thing.
The curious part about this argument is that it only works for God; that is, one cannot say that because a purple unicorn can be thought of, that that purple unicorn exists. This is a very circular argument that Anselm makes, as it assumes that God is a special being in that the argument works only for It, and not for other beings, but this itself presents a problem because, for the argument to work, one has to assume that God exists to be a special being. This is pointless, of course, because the whole point that Anselm was trying to prove was that God exists; in order to do that, the argument cannot presuppose that God exists.
Anselm also goes on to write, “Thou so truly art, then, O Lord my God, that thou canst not even be thought of as not existing,” (Ans., Prosl.), and, “no one who understands what God is can think that God does not exist,” (Ans., Prosl.), both of which are quite false, as atheists believe exactly what Anselm is saying cannot be believed. Perhaps a rebuke by Anselm might be that these atheists then do not understand God, otherwise they would believe in It, but that presents a problem as well. That being, can any human understand God? It seems very hubristic of Anselm to demand understanding of deity, and ridiculous of him to expect that one must understand said deity before one can believe in it. This is because of the reasoning that one cannot expect understanding of a thing before one is certain of that thing’s existence; we need existence of an entity before we can truly understand it, because true understanding of a thing cannot come from mere ideas about the nature of that thing.
Aquinas finds fault with Anselm, and sums up Anselm’s argument’s main problem nicely:
Yet, granted that everyone understands… this word “God”… nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this is precisely not admitted by those who hold that God does not exist. (Aq., Summa Theol., Art. 1, Repl. Obj. 2)
Another response to an error in Anselm’s logic is, “because we do not know the essence of God, the proposition is not self-evident to us,” (Aq., Summa Theol., Art. 1, Answer).
Aquinas does the best job of proving the existence of God in his five proofs, and the first of these makes the most rational sense. He states that everything that is set into motion must have been moved by some previous action, and so on back in time. However, this cannot go on infinitely, because that would mean that there was no first mover and thus that there was no beginning to movement. That is clearly not the case, as there is movement now, action and reaction now, and so there must have been a beginning.
To have a beginning, though, there must be a mover that did not require a mover itself. This mover-without-prior-mover we call God. One might ask what makes God special, and not requiring a mover Itself, but that is looking at the situation from a wrong angle: the definition of what God is said to be makes It special. It is called God precisely because It does not require a mover for It to move, precisely because It is special. If what we called God did require something to move It before It could be set into motion, then we would be mistaken in calling It God. We would apparently not have gone back far enough into the chain of movers and the objects they move. This leads to the interesting idea that if we cannot see that God exists, it is our own shortcoming, and certainly not one of God’s due to Its lack of existence.
Aquinas’ second proof is very much like his first; he seemingly only exchanges the words “mover” and “moved” for “cause” and “effect.” In his third proof, there is a small problem, and it is this: Aquinas states, “We find in nature things that are possible to be and not to be… But it is impossible for these always to exist, for that which is possible not to be at some time is not,” (Aq., Summa Theol., Art. 3, Answer). However, just because it is possible for something to happen does not mean that it must happen. The possibility is there, and so the potential for occurrence is there, but it is not a certainty that the possibility will ever be fulfilled. It is important that Aquinas is wrong in claiming what he does, for he goes on to say, “Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist… which is absurd,” (Aq., Summa Theol., Art. 3, Answer). Because it is only a possibility that at one time nothing was in existence, and not a definite certainty, then the rest of his argument is null.
Aquinas’ fourth proof is not as well written as his first, but it does stand on its own. An analogy for what he seems to be claiming can be found in a set of numbers: for any given set of numbers, there must be a highest number. Even if there are several numbers of the same highest value, those several are the highest values. God can be said to be the highest value in the set of numbers that is all creation. Aquinas talks of the gradation of things, and uses the words, “less good, true, noble, and the like,” (Aq., Summa Theol., Art. 3, Answer) to describe the relationships between objects. He says that God is “something which is to all beings the cause of their being, goodness, and every other perfection,” (Aq., Summa Theol., Art. 3, Answer), i.e. the highest term in our set of numbers.
The fifth proof that Aquinas delivers is done nicely as well. Aquinas writes of beings that have no obvious intelligence and yet always they act toward the same end. Repetitively, without fail, these beings reach the same result each time they are acted upon in the same manner. (Chemical elements and their properties come to mind, for example.) Aquinas states that such order in clearly mindless entities cannot be by chance, or by any desire of the mindless entities themselves. Any direction that these entities possess must come from an intelligent being, and that being is God.
Aquinas’ argument makes sense when one considers, from what other being would such direction come? Human beings may throw together specific chemicals, but we do not make those chemicals react how they do. Since we are the most intelligent species of which we know, and we are definitely not causing particular elements to behave as they do without fail, there must be some higher power that is.
It is therefore by logical necessity that God must exist. While not all of the arguments set forth by Anselm and Aquinas were foolproof, only one good argument is necessary to prove God’s existence if that one cannot be disproved. Perhaps in the future when new discoveries come to light, we may find reasoning that can beat even the sturdiest of Aquinas’ five proofs, but for now, we must concede that God does exist, at least until we can prove It does not.