Philosophy 1600 - Honors Seminar

 

Topic 2: Cosmological Arguments for the Existence of God

Aquinas's "Third Way"

1. A Formulation of Aquinas's "Third Way"

(1)  Some things are generated and corrupted - that is, they come into existence and they drop out of existence.

(2)  Therefore some things have the possibility of not being.     [From (1).]

(3)  Whatever has the possibility of not being, at some time is not.

(4)  Therefore if everything has the possibility of not being, at some time or other nothing is.     [From (3).]

(5)  Nothing begins to exist unless there is a cause of its coming to be.

(6)  If at some time or other nothing is, then there is nothing now.     [From (5).]

(7)  It is not the case that there is nothing now.

(8)  Therefore not everything has the possibility of not being.     [From (4), (6) and (7).]

(9)  Everything which is necessary either has the cause of its necessity outside of itself, or it does not.

(10)  It is not possible to go on forever in a series of necessary beings which have a cause of their necessity.

(11)  Therefore there is something which is necessary, and whose necessity is not caused by any other thing.  [From (8), (9), and (10).]

(12)  Any being that is necessary, and whose necessity is not caused by any other thing, must be God.

(13)  Therefore God exists.     [From (11) and (12).]

2. Evaluating Aquinas's Third Way

Deductive arguments can go astray in either of two ways.  First, one of the premises of the argument may be false - or, at least, implausible.  Secondly, one of the inferences may be invalid (or fallacious) - that is, the truth of the relevant premises may not suffice to ensure the truth of the conclusion that is drawn from those premises.

One needs, therefore, to ask two questions about the above argument:

(1)  Are any premises false, or at least implausible?  (The premises in the above statement of the argument are the statements set out at steps (1), (3), (5), (7), (9), (10), and (12).)

(2)  Are any inferences fallacious?  (The inferences occur in the drawing of the conclusions set out at steps (2), (4), (6), (8), (11), and (13).)