Notes on Aquinas’ Third Way
3 min readAug 21, 2015
- By possible being — that which is “possible…to be and not to be” — Aquinas has in mind things that are subject to generation and corruption; they come into existence and pass away out of existence. Any corporeal substance is like this, since its matter is always in determination to many forms. Trees are possible beings in that they can cease to exist, and in so doing their matter exchanges one form — the form of a tree — for another — the form of lumber, the form of a coffee table, the form of cork, of cardboard, and so on.
- The crucial step in Aquinas’ Third Way is not the conclusion that a necessary being exists; this is merely the first stage of the argument. The crucial step lies in the distinction between what is necessary “of itself” and necessary “of another”. Aquinas held that some features of the universe, such as celestial bodies, exist necessarily, so he would not have been surprised to find that our current physics postulates things that also exist necessarily. But celestial bodies, as well as any other postulate of physics, can only be necessary in a derivative sense: their necessity is ultimately caused by something extrinsic.
- The Third Way rests on the hypothesis of an infinite world, precisely because anyone who denies the existence of God will presuppose that the world is without beginning. The atheist will hold that what is of infinite duration does not require a cause, which is well true if the cause he has in mind is a temporal cause. But nowhere does Aquinas argue that proving God’s existence is a matter of dialing back the world clock until we find God at t=0. Instead, Aquinas holds that the notion of infinite duration is crucial to showing, first, that not everything is metaphysically possible (contingent), and second, that not everything is metaphysically necessary in a derivative sense. With regards to the first, Aquinas argues as follows: suppose that all of reality is comprised of possible beings, beings that not only come into existence, but have an intrinsic tendency to go out of existence, a metaphysical instability, if you will. Suppose further that the world has always existed. Since every possible being tends, over time, to go out of existence, then, given an infinite amount of time, this metaphysical instability will play out for every possible being; indeed, it will have already been realized, such that no world would exist now. But a world does exist now. So, not everything is subject to generation and corruption.
- Again, the sense of ‘possibility’ active in the Third Way is one of metaphysical possibility, not logical possibility. The latter refers to any state of affairs that doesn’t involve a contradiction, and hence it is tied to the modal notion of ‘possible worlds’.
- A being is corruptible if there is within its essence a tendency to go out of existence, a disposition to non-being. Conversely, a being is incorruptible in whose essence no such disposition exists.
- If something comes into being — that is, if some being is generated — this can only be A) as a result of itself, B) as a result of nothing, or C) as a result of some extrinsic cause. If we say that something can bring itself into existence, we are affirming the absurd position that something can both be and not be, at the same time, and in the same respect; for such a thing must first be if it is to bring itself into existence, and yet not be if its existence is to be caused in any real sense. In the former case, it is clear that if a thing already exists, it would not need to bring itself into existence; but that which begins to exist at some time did not. In the latter case, it would have to be said that what comes into existence does so from nothing, otherwise we would have to concede the opposite, namely that it comes into being from something. But to suggest that what begins to exist has no cause of existence is to render that thing’s existence unintelligible; it is to plumb the depths of dogmatism and hold, before all reason, that there is no explanation for its existence. So the only reasonable account left is C, namely, that which comes into being does so as a result of some extrinsic cause.
- Whatever is physical is, by definition, not necessary. For what is physical is a composite of matter and form, and both must be conjoined in a substance from without.
