The so-called 'Five Ways' are the classical 'proofs' of the existence of God, and they are set forth by Thomas Aquinas in his Summa Theologiae, Book I, Question 2, Article 3.

According to the tradition which these five ways represent, the person who genuinely seeks God finds certain 'ways' or 'proofs' of God's existence. These are not proofs in the sense familiar to us from the sciences, nor are they 'knock-down' proofs. Instead they are in the nature of 'converging arguments'.

Change

The first way is based on movement, understood in the wider sense of change. One notices that there is movement, or change, all around us. And, thinking on it, everything that changes or moves is moved by another. Going back to find earlier movers in the chain, it is not possible to go back to an infinity in the number of those that move, or are movers, because then there would be no reason for the movement. Therefore, Aquinas writes, there must be a 'prime mover' that moves everything else, and is moved by nothing. This must be outside the world of change, and it is what we call God.

Cause and effect

The second way is based on cause and effect. Everything has a cause, and there is nothing in the material world that is its own cause. However, one cannot have an infinite series of causes because that would lead to an absurdity: if there were no first cause there would be no intermediate causes, and hence no effects. Hence the first cause is what we call God.

Contingency

The third way departs from the observed fact that everything in the universe bears this mark, that it is contingent. In other words, it exists, but might very well not have existed if some other thing had not been what it was or done what it did. The effect of this is that each thing's existence is contingent on something else; it received its existence from elsewhere. It has existence, and can pass it on, but did not originate it. It is impossible to conceive of a universe consisting only of receivers of existence, for without something that is the source of its own existence, there would be nothing in existence at all. Even an infinite chain of beings would not account for existence. Therefore, so that there be beings that are contingent, there must be something necessary of itself and not through another. This necessary existence is what we call God.

Gradation

The fourth way considers the grades of perfection that there are in things. One sees in beings that some are more or less good, more or less truthful, or noble than others. But this measure or 'more' or 'less' is attributed to things according to their closeness to a maximum. Therefore there must exist something that is most truthful, noble and good, and thus supreme. The thing that is best in whatever quality or genus or category is the cause of all those that are of that quality, for example as fire is the source of heat. There exists something that is for all things the cause of its being, its goodness and all of its perfections, and we call this God.

Order

The fifth way departs from the order that one sees in the world, especially the finality or purpose that we see in living things. 'We see, in effect, that things that do not have knowledge, such as natural bodies, work for an end, as we see that they always or nearly always work in the same manner to attain that which is most convenient for them. But things that lack knowledge do not tend to an end if someone who knows and understands does not direct them, as an archer directs the arrow. Therefore there exists an intelligent being that directs all natural things to their end - and this we call God.'

Conclusion

In summary, these five ways or proofs take the facts of creation and its patterns (change, cause and effect, contingency, gradation and order) as their point of departure. Taken all together the converging arguments are impressive.