Let us now walk through Saint Thomas Aquinas’s
third way to prove the existence of God. How does the argument go?
All of the five ways begin from some observable feature of things in the world,
and we go from there to ask after a cause or reason for what is observable.
The first way begins from the simple observation that some things are in motion in the broad sense
of undergoing change and the argument goes on to ask why things are in motion. The second way
begins from the observation that efficient causes are at work in the world and the argument goes on
to ask why efficient causes are at work. The third way begins with the observable fact that
some things begin to exist and cease to exist. Blades of grass, dandelions, and apple trees,
for example, begin to be and cease to be. So, too, do cats and dogs and many other
ordinary things in our experience. They are subject to generation and corruption.
When we ponder things that actually begin to exist and actually cease to exist,
we can safely conclude that each of them is “able” to exist and “able” not to exist. Aquinas
calls things of this sort “possible beings.” The expression “possible beings” might be confusing,
since the term “possible” sometimes suggests something that merely might be but is not real
or actual – like an imaginary horse or a horse in another possible world. But in the third way
Aquinas is not using the term “possible” in this sense. Rather, he is talking about actual things,
but he wants to point out something specific about some of them. He wants to point out that
some of them are able to be and able not to be. Now, the world around us is full of possible
beings in this sense. Possible beings, in fact, are so common that we scarcely notice them
nor notice the marvel they contain. Each blade of grass, for example,
exists but does not “have” to exist. It is able not to exist. Each dandelion exists,
but does not have to exist. For a dandelion is the kind of thing that is able “not” to
be. So too with cats and dogs and many other things in nature. So, too, with you and me.
You and I exist, but do not have to. When we consider possible beings, and
notice that a possible being is but need not be, one might well ask: why does it exist at all? The
mind goes in search of a cause of the very being or existence of such things. For somehow we know,
if only vaguely, there must be some cause or explanation. The third way goes through two
main phases of reasoning in search of an answer. In the first phase, Aquinas says it cannot be that
all things whatsoever are possible beings. Why not? When it comes to possible beings,
he says, each of them at some point is not. In other words, pick any possible being you like,
at some point it doesn’t exist at all. It is clear enough that Aquinas thinks of possible
beings as essentially temporary beings. They either begin to be or cease to be.
Going one step further, Aquinas then says if “all” things were of the sort that are able not to be,
in other words, if all things were possible beings, then at some point nothing existed
in the world. However, if at some point nothing existed in the world, then nothing would exist
even now. But it is obviously false that nothing exists even now, so it cannot be that all things
are of the sort that are able not to be. In other words, not all things are possible beings.
There must exist at least one necessary being. This first phase of reasoning is admittedly hard
to understand. It seems to many people there are certain ambiguities and fallacies in the
argument. In fact, even the world’s foremost experts in the thought of Thomas Aquinas find
it hard to interpret just what he is saying at certain points in this phase of the argument
and why he is saying it. Scholars have offered many interpretations and defenses, especially
by looking into background principles that Aquinas might be taking for granted but leaving unstated.
One thing, however, is clear. This first phase of the argument, and its reasoning in terms of
time and the totality of possible beings, is a unique passage in all of the writings of Thomas
Aquinas. In other places, such as the Summa contra gentiles Bk. I ch. 15 and Bk. II ch. 15,
Saint Thomas takes up the question of why possible beings exist, and he reasons to an answer along
much simpler lines than in the third way. We will look at those arguments in other videos.
They will shed light on the larger question. Let us now turn to the second phase of the
third way. The second phase takes for granted the conclusion of the first phase: a necessary being
exists. At this point in the argument he is not thinking of necessary beings in terms of possible
worlds, but in terms of change and time. A necessary being, in Aquinas’s terminology,
is something that is not subject to generation and corruption. It is a permanent being. Such a being,
however, is not necessarily God. For Aristotle thought the celestial
bodies were neither generated nor corrupted. Perhaps with this in mind, Aquinas points out
that a necessary being either has its necessity caused from outside itself or it does not. If it
has its necessity caused from outside, then there must be another necessary being to account for it,
and the other necessary being either has its necessity caused from outside or not. If the
other one also has its necessity caused from outside of it, then there must be yet another
necessary being to account for it and so on. No matter how many necessary beings we multiply,
no such series really accounts for the necessary beings. So, there must be at least one being
that is necessary through itself. In other words, nothing else causes it to be or to
be necessary. Its very essence is to be. Aquinas then says “this everyone calls a
God.” Aquinas knew well that at this point he had not yet demonstrated all the attributes of
the one God that Christians know and love. Rather, the conclusion of the third way is
only the beginning of a longer meditation and investigation over the course of many
further questions in the Summa. The rest of the investigation will lead us to the one true God.
For readings, podcasts, and more videos like this go to Aquinas101.com. While you're there,
be sure to sign up for one of our free video courses on Aquinas. And don’t forget to subscribe,
like and share with your friends, because it matters what you think!