St. Thomas Aquinas listed what he saw as five intellectual proofs of the existence of God—proofs that were dependent on reason and observation, not the revealed word of God.
Aquinas and the First Way:
Aquinas recognized that for motion to take place, there had to be something that interacts with it to cause it to move. For a ball to move, for example, it must be struck by another object, for example, the foot of a child kicking it. The ball has the potential to move, but that potential cannot reach its actuality until something else acts upon it. Aquinas argued then, that as the original object that was moved needed to have something act upon it to move, so too does the second object have something act upon it. The boy swings his leg, which moves his foot which in turn moves the ball. And the chain continues backwards from there. He also recognized that without a first mover, the chain of cause and effect must, by definition, go eternally back. Since that idea is absurd to the ordered mind and is not consistent with observable evidence, there must be a first mover upon which nothing is needed to act to cause him to move. This, in turn must be an infinite being outside of creation and hence is God.
While it is not my purpose to go into a detailed critique of these proofs, it is important to point out what Aquinas is doing. It is clear from the language that this is designed to be an intellectual argument for the existence of a god, but it does not point clearly to the existence of the Biblical God. This proof could just as easily be applied to Allah, Odin, or Jupiter. The point is simply to argue that it is impossible to rationally look at our world without seeing the reality of a creator God.
Aquinas and the Second Way:
The second approach that Aquinas mentioned is similar to the first, but focuses on cause and effect rather than on potential motion being converted into actual motion. Every effect must have a cause, if you eliminate the cause you eliminate the effect. Once again, since an infinite series of cause and effect is irrational, the principle posits that there must be an original cause that in itself does not need a cause: hence God. Again, this does not posit the God of the Bible, or even a good and benevolent God for that matter, it only posits that a God exists who is the cause of all things and who is the effect of nothing.
Aquinas and the Third Way:
The third approach deals with a question of being and not being. Aquinas argued that from observation, the things around him had the possibility of being (or existing) or not being. The chair that you are sitting on exists, but it has not always existed. There was a time when the chair was not. He went on to observe that for something to move from not being to being, that action had to be brought about by something that was being. In other words, for the chair we spoke of earlier to come into being, it had to be manufactured. To manufacture something you must “be.” Something that does not exist cannot make something come into existence, the idea of such is nonsensical. Thus, all things that exist must be brought about by that which exists. Just as in the question of causation, there must be a first being. Yet, if that first being exists, he must necessarily not have the possibility of not being. In other words, as non-existence cannot bring about existence, the first being necessarily has to have always existed. And this entity that necessarily exists and cannot not-exist, is God.
Aquinas and the Forth Way:
Aquinas points out that we recognize that there are degrees of things. Some things are better than others; some things are shorter or taller or colder or hotter, etc… than others. And thus we rate them as good, better, and best. Yet, for us to have the idea that one thing is better than another, we must have a standard by which all things are measured and that can never be exceeded. That standard, then, is God. Note that this is not the suggestion that we get the idea of goodness or hotness from God, but simply that there must always be something that is more good or more hot than that which we are viewing and since there is a gradation, there must always be a top to the gradation that can never be surpassed. Such a top or asymptote, by definition, requires an infinite being, hence it must be God.
Aquinas and the Fifth Way:
Fifthly, Aquinas points out that there are entities in creation that have no consciousness at all, yet still act in a regular fashion and in such a way that it is beneficial to their continued existence. Trees, for example, have no consciousness of their own to direct themselves, yet they will sink their roots deeply into the soil to collect water, they will spread their branches wide to collect light for their photo-synthetic leaves, and they will drop seeds by which they may propagate their kind. Aquinas observed that since they act with some sense of direction in terms of self-preservation, yet are unguided by their own consciousness, they must be guided by the consciousness of another. This, once again, is the role of God.