The Five Ways of St. Thomas Aquinas, Part III: The Third Way


The Third Way – The Argument from Necessity

Building upon the Argument from Efficient Cause, the Third Way demonstrates that there are things that are non-necessary, (meaning that it’s possible for them “not to be,”) and which are caused by a necessary thing that “has to be.”

This argument says that whatever has been brought into existence has not always existed, and will not always exist. Thus, the existence of everything is contingent upon something outside of itself, a necessary thing, to bring it into, and keep it in existence. These things are possible, but not necessary.

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 [see Argument From Efficient Cause]. 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.

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.

This is a hard concept to get: the most fundamental forces and beings in the universe are “necessary” only so far as they cause the existence and actions of everything that isn’t necessary; they require and assume the existence of the entire universe, just as a DNA strand requires the entire cell in which it acts. For instance, without mass-bearing matter to influence space-time, there is no gravity; without space-time, matter can have no gravitational field. Gravity, then, can “not be”, so it can’t be uncaused. The God of the Third Way, then, is the one Being that cannot not be. We call this necessary and non-contingent being, God, “I AM WHO AM,” or Pure Being (Exodus 3:14).

From Anthony S. Layne, Managing Editor

I’ve been fortunate to work with Michele Boyer on this project. She asked to add this explanation under my name rather than hers.

Some skeptics have a tendency to think that if you throw a large enough number at a problem it will go away. For instance, many physicists are looking at a package of ideas called string theory. String theory postulates, among other things, that there may be as many as 10500 universes, and that the fundamental laws of our universe might not even apply to some or any of them. Many hope string theory will not only provide them a Theory of Everything — the Holy Grail of physics — but also finally do away with all this universal Creator nonsense.

But the sheer incomprehensibility of the number doesn’t take away the fundamental difficulty. In fact, multiplying the number of universes to such a fantastic degree merely underscores the problem. If the fundamental particles and forces don’t have to exist in all universes, they don’t have to exist — period. If Planck’s constant doesn’t have to apply across all possible universes, it can’t exist apart from the universes to which it does apply. If anything didn’t in any sense have to be, then it can’t be uncaused. The God of the Third Way, then, is the one Being that cannot not be.


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