3. The Argument from Time and Contingency

The third argument of the Five Ways by Saint Thomas Aquinas.  I can think of no better way to describe this than the step by step explanation of Peter Kreeft and Ronald Tacelli in The Handbook of Catholic Apologetics, pages 57-58.  (emphasis mine)

1.  We notice around us things that come into being and go out of being.  A tree, for example, grows from a tiny shoot, flowers brilliantly, then withers and dies. 

2. Whatever comes into being or goes out of being does not have to be; its nonbeing is a real possibility. 

3.  Suppose that nothing has to be; that nonbeing is a real possibility for everything.

4. Then right now nothing would exist.  For

5.  If the universe began to exist, then all being must trace its origin to some past moment before which there existed- literally- nothing at all.  But

6.  From nothing nothing comes.  So

7.  The universe could not have begun. 

8. But suppose the universe never began.  Then, for the infinitely long duration of cosmic history, all being had the built-in possibility not to be.  But

9.  If in an infinite time, that possibility was never realized, then it could not have been a real possibility at all.  So


10.  There must exist something that has to exist, that cannot not exist.  This sort of being is called "necessary". 

11.  Either this necessity belongs to the thing in itself or it is derived from another.  If derived from another there must ultimately exist a being whose necessity is not derived, that is, an absolutely necessary being. 

12.  This absolutely necessary being is God. 

Scott Hahn in Reasons to Believe describes it this way:

"If some beings are contingent, there must be an ultimate being that is neither transitory nor derivative, but necessary- necessary in and of itself.  And it is that being we call God."