The third way: The way of CONTINGENCY
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to be corrupted, and consequently, it is possible for them to be and not to be. But it is impossible for these always to exist, for that which can not-be at some time is not.Therefore, if everything can not-be, then at one time there was nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist begins to exist only through something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence - which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has already been proved in regard to efficient causes. Therefore, we cannot but admit the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God. (Aquinas)
Things exist, but they could easily not exist! There was a time before certain things existed, and there will be a time when they no longer exist. There must also have been a time when nothing existed. This means that objects that exist have contingent existence, which means they could or could not exist. For Aquinas, the only thing which has always existed is God (who would therefore have necessary existence). Aquinas saw no way to explain how anything was here, unless something existed prior to it. Thus for him, if God did not exist then nothing else would.
The essence of Aquinas’ argument in bold above, assumes that God created the world ex nihilo (out-of-nothing). It was literally caused to be here from no other natural cause. Of course this could be challenged today by Cosmologists who argue that although there was no form to matter before the “Big Bang”, there was still something prior to the ‘Bang” which led to it occurring (E.g. Quantum fluctuations).
The fourth way: The way of GOODNESS
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble, and the like. But more and less are predicated of different things according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest, and, consequently, something which is most being, for those things that are greatest in truth are greatest in being... Now the maximum in any genus is the cause of all in that genus, as fire, which is the maximum of heat, is the cause of all hot things, as is said in the same book. Therefore, there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God. (Aquinas)
We see in the world degrees of perfection and goodness. Some things are really bad, some not so bad, some things are better than others. Aquinas argues that we only know things are 'degrees of x' (i.e something is really bad or not so bad) because we compare them to the best in any group (or genus) of things. So something not so bad is being compared to something that’s really bad!
Now in terms of morality, as humans have the capacity for both good and bad deeds they cannot logically be the source of Goodness (i.e. the most Good thing), because if they were they would not do bad things. Therefore, the maximum in the genus of morality must be something non-human, and not in the world (because this too is not completely good), which leaves us with God as the most perfect being, and the 'first cause' (or source) of all goodness and perfection. Of course, it might be argued that this is not so much an argument for the existence of God, but for the existence of some standard of morality. We could could say there is an ultimate standard of Goodness, without requiring us to posit the existence of God. Far more compelling (in terms of any moral argument for God’s existence) is to use God’s existence as as way of explaining why humans are something compelled to act in an altruistic manner, if there is nothing more than this world or life.
The fifth way: The way of DESIGN (or teleology)
The fifth way is taken from the governance of the world. We see that things which lack knowledge, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that they achieve their end, not fortuitously, but designedly. Now whatever lacks knowledge cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is directed by the archer. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God. (Aquinas)
Aquinas' argument here is basically suggesting that inanimate objects (E.g. Planets), could not have ordered themselves on their own (i.e. got themselves into the orbits they have), because they lack the intelligence to do so. As the planets are aligned so perfectly, this means it must have been done by a Being with the ability and intelligence to do this. Although humans are intelligent and can explain planetary motion, they cannot move planets, so that leaves us with God (who Aquinas believed could move planets if God chose to do so).
From simple observation nature suggests a realm of order and purpose. This is the basis of science. The world is able to be observed, understood and explained. For Aquinas, anything which has in it a sense of purpose and order requires a 'guiding hand'. For instance, an arrow only hits the target because it has been fired by an archer. Thus if nature appears ordered, it must have a 'guiding hand', who Aquinas believed was God.
A weakness with Aquinas’ Fifth Way is that for science to be capable of observing, understanding and explaining the world we live in, we must first assume a sense of order and stability about things. The idea that there is some Deity acting in and on the world raises questions in terms of science’s ability to test, examine, and explain things. For instance, we cannot easily talk about the ‘laws of nature’ being constants, and have a God acting in the world doing miracles for example. So the scientific enterprise has gradually removed the notion that God acts in nature, to one where nature acts independently of any Divine Being. In the end, the more science provides us with an explanation of how and why things are the way they are, the less we find any need for a Divine Creator.