Thomas Aquinas belongs to those who reject the ontological argument because he saw the argumentative side in it, which indeed must be rejected and is not valid.
The same of course is true of Duns Scotus. I don’t need to go into him at all. He emphasizes this even more.
But now in order to fill the empty space which was produced by the falling down of the ontological argument, and also. in Thomas, by the principle of the immediate awareness of the Divine in man, he had to do something else – I spoke about this point yesterday – namely, to find a way from the world to God. The world in itself is not the first, but it is the first which is given to us, he says. This is just the opposite of what the Augustinian and the Franciscans said: the first which is given to us is the principles of truth in us, and only with their help can we exercise the function of doubt, etc. Even the skeptical function is based on the spirit of truth in the depths of the mind. Thomas denied this. So he had to show another way: the cosmological way, which says that God must be found from outside. We must look at our world, and we find that our world is such that by logical necessity it leads us to the estrangement of a highest being. He has five arguments for it, which one should know because they appear again and again in the history of philosophy: 1) The argument from motion: Motion demands a cause. This cause itself is moved.
So we have to go back to an unmoved Mover – which we call “God.” – It is an argument from movement in terms of causality. To find a cause for the movement in the world, we must find something which itself is not moved.
2) There is always a cause for every effect, but this cause is itself an effect of a prior cause. So we go back from cause to cause, which would bring us into an infinite regression, and in order to avoid this we must speak of a First Cause. Now the “first cause” is not the first cause temporally, according to Thomas, but it is first in dignity; it is the cause of all causes.
3) Everything in the world is contingent. It is not necessary that it is as it is. It might have been otherwise. But if everything is contingent, if we can make disappear into the abyss of nothing everything that is, because it has no necessity to be, then this leads us back to something which has ultimate necessity, and from which we can derive all the contingent elements.
4) There are purposes in nature and man, but if we act in terms of purpose, we ask: for what? And if we have reached that, then we again ask: for what is that? We need a final purpose, an ultimate end behind all the means. The preliminary purposes become means when they are fulfilled, and this leads to the idea of a final purpose, of an ultimate meaning, as we would perhaps call it today.