As written by Kevin Brown (certified philosophical counselor )
There are two main philosophical arguments which present evidence for the existence of God. These are the cosmological argument and the teleological argument.
St. Thomas Aquinas advises us to begin with a consideration of the existence of God. As he points out in the Summa Contra Gentiles, "if we do not demonstrate that God exists, all consideration of divine things is necessarily suppressed." Given his admiration for Aristotle, it should come as no surprise that Aquinas is an empiricist. While he concedes, as many did in the Middle-Ages, that the notion that God exists is self evident, humans require some sort of physical evidence to understand this. In other words, our intellect is not sufficient to understand the self-evident nature of the proposition "God exists." We need empirical evidence.
Aquinas' proof is called the
cosmological argument for the existence of God because its based on the existence of the cosmos; that is the universe. But before we see how the existence of the cosmos can be used as evidence for the existence of God we need to address a fundamental question about the cosmos' existence itself. Question: Was the cosmos created or has it always existed? From Aquinas' viewpoint this question could not be answered philosophically. Sure, we could offer evidence in favor of one or the other view but the evidence would not be conclusive. So either way we begin we must begin with an assumption.
The question is which assumption should we take? We could, using revelation, take the assumption that the cosmos was created. After all, the Bible tells us this. But then we run into a problem. We are trying to prove that God exists and we're using the Bible as a basis for this proof? Isn't this suspicious? After all, the Bible is only valid if we can show that it's the inspired word of God which we can only do if we can prove God exists. Using the Bible as evidence for our proof means we're presupposing that God exists. We're begging the question. So in spite of what revealed theology tells us we should not assume the universe was created.
Besides we run into another problem doing this. If we assume the universe was created and it turns out that it really is eternal what happens to our proof based on the assumption of it being created? It's worthless. After all, our proof will probably attempt to show that God created the universe. But if the universe is eternal it doesn't need a creator. No, the best way to begin is by assuming that the universe is eternal. That way we make no theological assumptions of God's existence to begin with and if it turns out that the universe really was created our proof is not ruined. In fact, it becomes stronger. By starting with the more difficult of the two assumptions Aquinas is making his proof stronger. If we can prove that God exists and is the cause of the eternal universe it will be easy to adapt this proof to God's existence if the universe was created. So Aquinas is proving that the universe needs a cause and that cause must be God. Even an eternal thing needs a cause (though not a creator; there's a difference).
Given all of this the structure of the proof contains four premises. Two of them Aquinas says should be self evident, the other two can be explained and if all four are true then we can conclude that the cosmos must be cause by something outside of space and time, something uncaused and ultimate. Which Aquinas maintains is God. Here are the four premises:
1. The universe exists.
2. It could not be the cause of itself.
3. It could not come from nothing.
4. It could not be an effect in an infinite series of causes and effects
The basic idea is that if we can show that these premises are true we can infer that God must exist. What Aquinas is doing is showing that the universe needs a cause and there are only so many options.
1. It could be the cause of itself. But premise two says this is impossible. The reason why is pretty easy. The cause must always precede the effect. But if the universe was the cause of itself it would have to exist before it existed! This, however, is impossible. Something can't exist before it exists. And given that the universe is eternal before doesn't really apply anyway.
2. It could come from nothing. But premise three says this is impossible. The reason why should be obvious: you can't get something from nothing. As Aquinas put it, ex nihilo, nihil fit . "From nothing, nothing comes." Anyway, the point is that it's impossible for the universe to have arisen from nothing.
Given that we're assuming its eternal could the universe have been the result of an infinite series of causes and effects? It seems unlikely for the following reason. Suppose I said to you "don't come back to class until you've read an infinite number of books." Are you ever going to return to class? Of course not! You can't read an infinite number of books. The point is its impossible to get through an infinite series. So if the universe were the result of an infinite series of causes and effects we would never see the final effect (the existence of the universe). We'd be waiting forever for the universe to exist. Given the fact that the universe exists (premise one) we can infer that the series of causes and effects is finite. In other words, there must be a first cause.
This is likely to seem confusing since we've said that Aquinas assumed the universe was eternal. How can an eternal thing have a first cause? The confusion can be clarified if we remember that cause does not necessarily imply creator. Aquinas is not arguing that God created the universe, but that the universe needs a cause for its existence. And more than this, it needs a cause for its continued existence through time. Another way of putting this is to say that the universe needs a sustaining cause.
There is no objection to the possibility that an infinite series extend backwards in time since Aquinas is assuming that the universe is eternal. But, could there be an infinite series of causes and effects in a single moment in time? This is the problem and the answer is no. So what Aquinas is objecting to is not a temporal series but a hierarchical series. The reason this series cannot be infinite should now be clear. But perhaps the reason why the beginning of this series must be God isn't. For Aquinas this is because the universe is contingent and requires a necessary being to exist to sustain it. Furthermore, as we have seen the cause must also be uncaused and outside of space and time. What other being could this description apply to except for God.
As I said, this derives from Aquinas' supposition that the universe is contingent; meaning it happens to exist but could also not exist. He argues in the following way. Since each thing in the universe is contingent it follows that the universe itself is contingent. Contingent beings come into and go out of existence. So it seems reasonable to suppose that at some point in the past nothing existed in the universe. But, if this were true there would be nothing in existence now; remember you can't get something from nothing. So therefore, there must be a being whose existence is not contingent but necessary and this of course is God.
Natural theology was given its clearest modern expression with the 1802 publication of Natural Theology by William Paley. In this work he provides us with an elegant version of the teleological argument for the existence of God ; also known as the argument from design.
Suppose, says Paley, we were walking in the forest and happened upon a stone in the path. We probably wouldn't think twice about it and go on about our way. But, suppose that further on we were to happen upon a pocket watch. Clearly our reaction to this would be different. It seems to be out of place. More than that, when we inspect it closely we recognize that it's a mechanism designed to do a specific job. We note its intricate workings and speculate that such intricacy could not have naturally occurred. It must be the product of design and intelligence. The existence of the watch implies the existence of a watchmaker.
Now, "were there no examples in the world of contrivance except that of the eye, it would be alone sufficient to support the conclusion which we draw from it, as to the necessity of an intelligent Creator." In other words, if the watch requires a watchmaker so too the eye must require an intelligent designer. And given the immense complexity of the entire universe and all its intricate working parts, the Creator must be infinitely intelligent and powerful. This constitutes Paley's proof of the existence of God.
In the rest of Paley's work he proceeds to show how each part of the universe, as a separate creation, can be seen as evidence for God. In parts, Natural Theology reads like a science textbook but this is simply Paley's attempt to offer as much detailed evidence as possible for his argument. Given the 18th century scientific revolution it seems only natural to apply the reasoning of science to theology.
There are, of course variations on each of these arguments as well as criticisms to consider. But, this answer has perhaps been long enough already!
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