In discussing St. Thomas Aquinas’ Third Way, Mackie attacks the idea that something which could possibly not exist will eventually not exist. I’d agree with Mackie that this isn’t a logical necessity unless one accepts the Aristotelian views of matter and essence. Still, that hardly makes Mackie’s view the most likely possibility.
And, without opening the massive discussion about the Aristotelian philosophical system, this is one more case of Mackie arguing from possibility. It is certainly much more likely that something which is contingent will eventually go out of existence.
St. Aquinas argues that, barring divine assistance, the universe simply can’t go on forever. This is not, it should be clear, the argument that the universe began to exist. Aquinas, rather famously, thought this couldn’t be proved to anyone who rejects the authority of scripture. Rather, it is the argument that an eternal universe (as nearly all atheists claimed the universe was until very recently) would need something external to sustain it. And, once one works out what sort of thing could do this, one is left with theism.
And science seems to be catching up to philosophy here in discovering that we are headed for a heat-death. Even scientists are saying that, baring divine intervention, things aren’t going to last forever. As such, I’m not sure why (other than trying to unfairly discredit the argument) Mackie wants to argue that Aquinas is wrong to suppose that things can’t last forever.
This is doubly strange in that he follows this with the suggestion that the universe doesn’t need to sustain itself forever, because it, in fact, had a beginning.
And Aquinas, I think it is fair to say, would take that “objection” as a major concession. If one is going to assert that the universe came into existence in the finite past, then one has to contend with Kalam-style arguments for God. No self-respecting atheist would have conceded that the universe was past-finite before the twentieth century.
This seems like more of Mackie’s characteristic scattershot approach, where he shifts between contradictory views as the momentary need arises. It simply isn’t a logical, systematic refutation.
Even worse, Mackie’s argument against the Second Way, you will remember, was that it is “dubious” to claim that there are not actual infinite chains of causation. But this, given the modern cosmology that leads Mackie to suggest that the universe is past-finite, is far from dubious. Rather, a finite number of operations and causes is exactly what scientists have found to be the case.
Mackie can’t simply demand that we remember this science when Aquinas (purely for the sake of argument) shows that not even an eternal universe could exist without God, then expect that we forget this when faced with arguments (from Aquinas and others) based on the idea that infinite chains of causation don’t exist.
This “heads I win, tails you lose” approach is, to me, one of the surest signs that theism has the stronger claim on truth. It can answer either set of objections, and it is only by vacillating between the two that Mackie can make his atheism seem plausible.
In the end, that strikes me as rather desperate.