St. Thomas Aquinas’ “Third Way”

(Note: this is a brief excerpt of Alvin Plantinga’s  summation of St. Thomas Aquinas’ well known “Third Way.” Taken from God and Other Minds by Alvin Plantinga, p. 5-6)

(a) There are at present contingent beings (“things that are possible to be and not to be”).

(b) Whatever can fail to exist, at some time does not exist.

(c) Therefore if all beings are contingent, then at one time nothing existed–from (b).

(d) Whatever begins to exist is caused to begin to exist by something else already existing.

(e) Therefore, if at any time nothing existed, then at every subsequent time nothing would exist– (d).

(f) Hence if at one time nothing existed, then nothing exists now– (e).

(g) Hence if all being are contingent, then nothing exists now– (c), (f).

(h) Therefore, not all beings are contingent– (a), (g).

(i) Hence there is at least one necessary being– (h).

(j) Every necessary being either has its necessity caused by another being or its necessity in itself.

(k) It is impossible that there be an infinite series of necessary beings each of which has its necessity caused by another.

(l) Therefore there is a necessary being having of itself its own necessity, and this all men speak of as God– (i), (j), (k).