It is not clear to me that Aquinas’ third way was that much different from the modal form of the ontological argument
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 corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by 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 been already proved in regard to efficient causes. Therefore we cannot but postulate 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. From
newadvent.org/summa/1002.htm
Aquinas is claiming that the existence of contingent things leads logically to necessary things and if necessary things are possible they need to have been caused by other necessary things or by their own necessity. If by another, then these would only exist necessarily by a kind of “dependent” necessity from whatever they depend on. This dependency leads logically to the necessity of one absolutely necessary Being upon which all other necessary and contingent beings depend.
It is not clear to me how this is fundamentally different from saying, as the modal form of the ontological argument does, that if an absolutely necessary being is possible, i.e., one than which no greater can be conceived and that all others are dependent upon, then this necessary being must, of necessity, i.e., by definition, exist, which is exactly what Aquinas is claiming: a necessary Being is a presupposition for anything else to come into existence.
Both seem to be perspectives of the same basic concept, necessity.
By the way, I think that Knox is making an error when he claims,
All these efforts at the solution of the problem really depend for their
plausibility on a postulate which we do not grant–namely, that it would
have been impossible for the human race to infer God’s existence from his
creatures. If this were true, then it might be argued that the notion of
God must be an idea directly communicated to our minds.
The ontological argument does not presuppose that it is impossible to infer God’s existence from his creatures. This is not an either/ or kind of proposition. It is logically possible that both inductive and deductive proofs are possible. Having a deductive proof does not make inductive proofs impossible. Something can be true definitionally that can also be shown to be true inductively.
Knox’s references to the Church’s “discountenance” towards ontologism is quite a different matter than an ontological proof of God’s existence. The ontologism referred to by Knox concerns the Council of Vienna’s (1311-12) condemnation of the doctrine of the Begards who maintained that we can see God by our natural intelligence, that the being (ontology) of God can be
wholly conceptualized through our natural intelligence.
The ontological argument is far from being a leap into ontologism. It is a claim about the fact of God’s existence, not the capturing of the ontology of God in conceptualized form.
See
newadvent.org/cathen/11257a.htm on ontologism.
