1. Responses to Aquinas
Peter Vardy sees Aquinas’s third way (from contingency and necessity) as the most important.
The world is made of contingent things (things subject to change and decay)
Contingent things need not exist. They depend on something other than themselves for their existence.

2. If all things were contingent then there would be a time when nothing contingent existed.
If there were a time when nothing existed, then without there being some (non-contingent) agency to bring contingent beings into being, there would still be nothing.

3. Everything that is contingent is dependent on something else for its existence.
Each contingent thing must be preceded by something else to bring it into existence. Yet this sequence cannot stretch back indefinitely.
There must therefore be a non-contingent agent which is responsible for bringing the contingent universe into being. Aquinas refers to this being as God.

4. Criticisms Several questions are raised
Why not an endless series of causes?
Can there not be something which has always existed, but which may cease to exist in the future?
Why should the chain of causes lead back to a single origin – might there not be a whole collection of separate origins?

5. Vardy claims that Aquinas tries to establish dependence, rather than a stretch back in time.
Aquinas based his argument on a contingent universe – but in some ways the universe is not contingent. The matter and energy in the universe are eternal, though they are distributed throughout the universe in different ways at different times.
Developments in modern physics have begun to consider the idea that matter may not be eternal.