As with Paley's design argument, Way 3 is a posteriori and inductive so it is based on observation. It is based on the particular observation that all things we see in the universe are contingent: they are moved, changed and caused. From the observation of contingency Aquinas concluded that something must exist necessarily

Contingency = In the first part of the argument, the core of the argument is that if everything is contingent, then at some time there was nothing. No contingent being is everlasting so there must have been a time where nothing existed. If there was a time where nothing existed then nothing would exist now because "out of nothing, nothing can come" but of course vast numbers of contingent things now exist

Infinite Regress = In the second part of the argument Aquinas rejects the idea that there may be an infinite series of caused necessary beings. That would be absurd because then there would be no ultimate cause of the series. So there must be an uncaused necessary being who sustains all caused necessary beings and all contingent beings. This is God

Criticism 1 (Russell) - Way 3 commits the Fallacy of Composition = Russell aims the criticism at Way 2 but it can also be applied to Way 3, which goes from 1) everything in the universe is contingent, to 2) the universe as a whole is contingent. This is fallacious as we can claim that 1) everything in the universe is contingent, but 2) the universe as a whole is necessary.

Criticism 2 (Hume & Russell) - the words 'necessary being' are meaningless = Having already rejected the claim of the ontological argument that 'God exists' is logically true, Hume now rejects Aquinas' claim that God is a 'necessary being' because he thinks that this is the same claim

Criticism 3 (Hume) - the universe itself may exist necessarily = Aquinas accepts this but argues that the universe could only exist necessarily if it was brought into existence by an 'uncaused' necessary being. Who is right here depends on what you think is the most probable explanation for the universe: a necessarily existent mind or necessarily existent matter

Criticism 4 (Russell) - the universe exists as an unexplainable brute fact = Against Russell, if the universe is unexplainable, it seems very odd that science works on the opposite principle

Some object that there could be a group of necessarily beings rather than just the one: however Aquinas argues that unless there is one being who contains within itself the reason for its own existence then the existence of anything is inexplicable

Some object that there could be a group of uncaused necessary beings, but Occam's Razor can be used to argue that this multiplies entities unnecessarily

Some object that there could be an infinite regress of contingent beings with no need for a first necessary being, but 1) this would still not explain why there is something rather than nothing, and 2) although we can have mathematical infinities, we have no evidence then an infinite past sequence can exist in the 'real' world