CFS-WB-CH04

The Third Way
Let us now look at Aquinas’ third proof, which deals with what we might
call the limited life cycle of things. Aquinas calls this way the argument
from possibility and necessity. It is possible for things to exist, and to
not exist. A tree, for example, grows from a planted seed, flourishes for
a time, but eventually dies, decays, and ceases to be. He notes that all
around us are things that come to be and cease to exist. Such beings do
not have to exist, and therefore, are not existence itself. If a being is not
existence itself, it does not exist through itself, but rather exists through
something else — it is caused by something else. If everything in reality
were caused by something else, then the whole of reality itself would
have to be caused by something else. There is, however, by definition,
nothing else outside the whole of reality. Thus, there must be a being in
the whole of reality that does not need a cause of its existence. If there
were no uncaused being in the whole of reality then the whole of reality
would not exist, which is plainly false. Therefore, there must be a being
which is uncaused — which exists through itself because it is existence
This artist depicts Christ’s
blessing of St. Thomas,
between Plato and Aristotle.
Triumph of St. Thomas Aquinas, “Doctor Communis”,
between Plato and Aristotle, by Benozzo Gozzoli.
76 Apologetics I: The Catholic Faith and Science
© Magis Center