RE

St Thomas Aquinas (1224-1274) was a well-known monk, philosopher and theologian.

Aquinas offered five ways to prove the existence of God, of which the first three are forms of the cosmological argument - arguments from motion, cause and contingency.

Aquinas was influenced by Aristotle's approach to causation.

This is the argument from motion or change (the two terms are equivalent as if you move from A to B then you have changed).

(1)    All things are in a state of change.

(2)    Everything is a secondary mover.

(3)   If all things are secondary movers then there must be infinite regress.

(4)   If (3) is correct then there is no Prime Mover. Without a Prime Mover, there can be no secondary movers, therefore, (3) is false as infinite regress is impossible.  (The reductio ad absurdum technique.)

(5)   Therefore, there must be a prime mover. This we call "God".

"The chain of movers cannot go on to infinity because then there would be no first mover and consequently no other mover." Aquinas, Summa Theologica

This is the argument from causation.

(1)   There is an order of efficient causes.

(2)   No efficient cause can cause itself.

(3)   If there is infinite regress then there is no First Cause.

(4)   If (3) is true then there can be no subsequent causes.  Infinite regress is impossible.  (The reductio ad absurdum technique.)

(5)   Therefore there must be a first cause of everything. This we call "God".

• Aquinas does not explain why there cannot only be secondary causes.
• Some argue he commits the Infinite Regress Fallacy by saying that infinite regress is wrong. In nature around us, we have infinite series, so why shouldn't nature itself be an infinite series?
• It is too large a leap from First Cause or Prime Mover to God. (However the argument doesn't prove or set out to prove the God of Classical Theism.)
• Why should we make God the exception? Why not make the universe the exception?
• Who made God - why does God have to be the First Cause?
• The argument only works with an exception, therefore everything is a secondary mover except God, which is a contradiction. (But the very nature of the argument shows that there must be a contradiction because without a First Cause, nothing can exist.)
• Quantum Physics suggests that at a sub-atomic level there may be random movement and there exist things that do not have causes themselves.

This is the argument from contingency and necessity.

A contingent being is one that relies on something else for its existence.  A necessary being relies on nothing else for its existence.(1)   Things are contingent.(2)   If everything is contingent, there must have been a time when nothing existed.(3)   Therefore (using reductio ad absurdum) nothing can come from nothing.

(4)   Therefore, there must be a necessary being.

(5)   Every necessary being must have a cause either inside or outside of itself.

(6)   Imagine every necessary being had a cause outside itself.

(7)   Therefore (using reductio ad absurdum) if (6) is true, then there is no ultimate cause of necessity.

(8)   Therefore, there must be a necessary being which causes and contains all other necessary and contingent beings. This being we call "God". 

• Why can't there be overlapping contingent beings going back to infinity?  Also, causes may have more than one effect. The jump from (1) to (2) is a fallacy.
• There is no sense in which it proves the God of Classical Theism.
• It assumes that infinite regress is impossible.
• Some might say it is meaningless to ask what caused the universe.

• "If the material world rests upon a similar ideal world, this ideal world must rest upon some other; and so on without out. It were better therefore never to look beyond the present material world." Dialogues Concerning Natural Religion

Hume's point is that we must not look beyond this world to anything metaphysical - The answers are in this world, otherwise we will end up going back to infinity.

• The Fallacy of Composition. Hume says that Aquinas has made a mistake in the way this argument was put together. Just because there is a common property to a group doesn't mean that property must apply to the group. Just because every event in a series has a cause, doesn't mean the series itself has a cause.
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Hume's point is that we must not look beyond this world to anything metaphysical - The answers are in this world, otherwise we will end up going back to infinity.
 

• Hume also said that we have no experience of a universe being created and so we cannot talk meaningfully about it.

• Hume rejected the idea of necessary existence - every being, according to Hume is contingent.

• Hume questions why motion needs to have a starting point - in other words why infinite regression is impossible. Surely if there can be an understanding of a prime mover there can be an understanding of perpetual motion?