St. Thomas Aquinas

The Five Ways from Summa Theologica

1st Way-From Motion (change)

1.  We can see that some things in the world are in motion (the change from potential to actual). E. g., Wood is potentially hot, fire is actually hot.

2.  Nothing can be both potential and actual at the same time, in the same respect.  (E.g., the wood cannot be both potentially hot and actually hot at the same time.)

3.  Therefore, it is impossible that a thing can be both mover (the thing bringing about the change) and moved (the thing changed), in the same respect, at the same time.

4.  Therefore, whatever is put in motion must be put in motion by some other actualized thing, and that by some prior actualized thing, and so on, and so on.  (E., g., the wood cannot set itself on fire.  The wood must be set on fire by some thing which is already hot.),

5.   No infinite regress of causes-otherwise there would be no first mover and so, no subsequent movers.

6.  Therefore, there must be a Prime Mover, Unmoved Mover, which everyone understands to be God

2nd Way-From Efficient Cause

1.  We see in the world an order of efficient causes.

2.  If a thing were to be its own efficient cause, it would be prior to itself (it would have to exist before it exists-it would exist and not exist at the same time) that’s impossible or absurd.

3.  So, nothing can be its own efficient cause. (by reductio ad absurdum)

4.  The first cause is the cause of the intermediate, and that of the ultimate cause.

5.  To take away a cause is to take away its effect.

6.  So, if it is possible to go to infinity with efficient causes, there would be no first efficient cause.

7.   If there were no first efficient cause, there would be no intermediate efficient causes.

8.  There are intermediate efficient causes (we see them in the world). These intermediate causes are effects of prior efficient causes (We just denied the consequent of 7.  By two steps of Modus Tollens we get-9 and 10)

9.  So, there cannot be an infinite regress of efficient causes.

10.           Therefore, there must be a first efficient cause, to which everyone gives the name of God.

3rd Way-From Possibility and Necessity

1.  In nature we find that things are generated and destroyed (corrupted).

2.  So, they are contingent (possible to be and not be)

3.  That which is contingent at some point does not exist (is not).

4.  So, it is impossible for contingent things to always exist.

5.  Therefore, if everything were contingent, at some point there would have been nothing. (nothing could have existed.)

6.  That which begins to exist is brought into existence by something which already exists (He assumes Ex nihilo nihil fit Latin, “Nothing can be produced from nothing” and what he has deduced in the 1st and 2nd ways.)

7.  Therefore, if at one time nothing were in existence, there would not be anything now!

8.   That is absurd. (So, If you assume that everything is contingent, you are led to the absurdity that nothing now exists but, of course things do exist).  Reductio ad Absurdum.

9.  (So, it can’t be true that everything is contingent)

10.              ( or, we could get there by saying ‘Something does exist now!’ By Modus Tollens denial of the consequent of 7, leads to “So, there was no time in which nothing existed.” )

11.              (“so, there was no time when nothing existed” is a denial of the consequent of 5. So, we could conclude ‘Hence, it is not the case that everything is contingent’ – another Modus Tollens.)

12.              Therefore, there must be a necessary being.

13.              No infinite regress of necessary beings

14.              Therefore, there must be a necessary being having its own necessity, and this all men speak of as God.