The 5 ways of Saint Thomas
The existence of God can be proved in five ways. The first and more manifest way is the argument from motion. (…) The second way is from the nature of efficient cause. In the world of sensible
The 5 ways of Saint Thomas
The existence of God can be proved in five ways. The first and more manifest way is the argument from motion. (…) The second way is from the nature of efficient cause. In the world of sensible things we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or one only. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate, cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The third way is taken from possibility and necessity. (…) The fourth way is taken from the gradation to be found in things. (…) The fifth way is taken from the governance of the world.
Saint Thomas Aquinas (1225-1274), Summa Theologica.
The ontological argument
The mind … looks around in all directions in order to extend its knowledge further. First of all, it finds within itself ideas of many things; and so long as it merely contemplates these ideas and does not affirm or deny the existence outside itself of anything resembling them, it cannot be mistaken. Next, it finds certain common notions from which it constructs various proofs; and, for as long as it attends to them, it is completely convinced of their truth. For example, the mind has within itself ideas of numbers and shapes, and it also has such common notions as: If you add equals to equals the results will be equal; from these it is easy to demonstrate that the three angles of a triangle equal two right angles, and so on. And so the mind will be convinced of the truth of this and similar conclusions, so long as it attends to the premises from which it deduced them. (…)
- The existence of God is validly inferred from the fact that necessary existence is included in our concept of God.
The mind next considers the various ideas which it has within itself, and finds that there is one idea- the idea of a supremely intelligent, supremely powerful and supremely perfect being – which stands out from all the others. (And it readily judges from what it perceives in this idea, that God, who is the supremely perfect being, is, or exists. For although it has distinct ideas of many other things it does not observe anything in them to guarantee the existence of their object.) In this one idea the mind recognizes existence – not merely the possible and contingent existence which belongs to the ideas of all the other things which it distinctly perceives, but utterly necessary and eternal existence. Now on the basis of its perception that, for example, it is necessarily contained in the idea of a triangle that its three angles should equal two right angles, the mind is quite convinced that a triangle does have three angles equaling two right angles. In the same way, simply on the basis of its perception that necessary and eternal existence is contained in the idea of a supremely perfect being, the mind must clearly conclude that the supreme being does exist.
- Our concepts of other things do not similarly contain necessary existence, but merely contingent existence.
The mind will be even more inclined to accept this if it considers that it cannot find within itself an idea of any other thing such that necessary existence is seen to be contained in the idea in this way. And from this it understands that the idea of a supremely perfect being is not an idea which was invented by the mind, or which represents some chimera, but that it represents a true and immutable nature which cannot but exist, since necessary existence is contained within it.
(…) Since, then, we have within us the idea of God, or a supreme being, we may rightly inquire into the cause of our possession of this idea. Now we find in the idea such immeasurable greatness that we are quite certain that it could have been placed in us only by something which truly possesses the sum of all perfections, that, by a God who really exists. For it is very evident by the natural light not only that nothing comes from nothing but also that what is more perfect cannot be produced by – that is, cannot have as its efficient and total cause – what is less perfect.
Furthermore, we cannot have within us the idea or image of anything without there being somewhere, either within us or outside us, an original which contains in reality all the perfections belonging to the idea. And since the supreme perfections of which we