4 Third Way
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 already been 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.
Davies, in introducing the Third Way states, “Aquinas begins the third way by saying some things come into being and pass out of being, that there are as he puts it, things which are ‘able to be or not to be,’ things which might be called ‘contingent’.” Aquinas is thinking how people are born and that they die. People’s birth is dependent upon the existence of the world and when they die, the world continues to exist. Aquinas seems to be distinguishing between necessary beings and contingent beings. He is thinking of contingent things as something that is born and that eventually dies. In other words, its existence is not necessary.
In the second part of the argument Aquinas says that if everything is contingent, the world would not exist, but the world does exist, therefore, there must be a being or beings that are not contingent. Davies explains, “The Third Way is arguing that not everything can be able to be or not to be because all such things depend on something for their being there, and without something not merely able to be or not to be there would be nothing at all.” If everything is contingent or depends on another for its existence, it seems for anyone to exist there must be a necessary being.
In the last part, Aquinas argues that there must be a necessary being whose existence is not dependent upon another. He asserts, “Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has already been proved in regard to efficient causes.” Aquinas seems to be saying that there are some necessary beings that are dependent upon others for its existence and there must be a necessary being who is not dependent for its existence on another. Feser thinks that the “only thing that could stop an explanatory regress of necessary beings would therefore be something whose essence and existence are identical, and who is a necessarily existing being precisely because it just is subsistent being or existence.” Feser seems to be drawing from Aquinas’ larger works how God’s essence and existence is the same. This is true for only God. This seems to imply essence to exist. This necessary being whose existence is not dependent upon another is what everyone understands as God.