One reason that might be given for the impossibility of an actual infinite regress of
simultaneous causes or movers is that if there were such a regress, there would be no
member of the regress that could be held to be morally responsible, a fit subject of either
praise or blame, for the initial event or object in the regress. But this can't be the right
reason, because not all causal explanations are forensic in the sense of giving an
individual who is to be praised or blamed for the effect. Maybe Thomas's underlying
intuition can be fleshed out by considering these two examples. In one, a group of boys
attempts to get into the movies free by having each boy point to the boy behind him as he
enters the theater and when the ticket taker stops the last boy in the group for the tickets
he claims not to know who these other boys are. (Richard Gale did it but Alexander Pruss
did not, as he grew up in communist Poland.) The last boy has to pay for himself, but all
the others get in free. Now suppose that the regress of boys pointing behind themselves to
another boy is infinite. Plainly, the theater owner would not be happy with this
arrangement, as he would never get paid, just as you would never succeed in cashing a
check if it were covered by a bank account that in turn was covered by another and so on
ad infinitum. A system of credit, like a succession of boys entering a theater, must
terminate with some actual cash. A second example involves a train of cars that
simultaneously push each other, such that the first car is simultaneously moved by a
second, and the second by a third, and so on ad infinitum. If the regress of movers were
infinite, there would be no explanation of where the oomph, the energy, the power to
move, comes from.
There is an implicit appeal to a version of the PSR to the effect that something cannot
come out of nothing. This can be made clearer by considering a circle of causes. Thomas
ruled this to be impossible for the same intuitive reason that he proscribed an infinite
regress of simultaneous movers or causes. Imagine that you meet someone who looks like
you would look in ten years. She claims to be your future self and to have traveled ten
years backward in time to give you instructions
end p.119
on how to build a time machine. Subsequently, you build one and then travel ten years
backward in time so as to inform your past self about how to build a time machine. The
intuitive grounds for Thomas's rejection of the possibility of this closed causal loop is
that it violates the PSR, for there is no answer to the question of from whence came the
knowledge of how to build a time machine. Similarly, there is no answer to the question
of from whence came the power to move an object or causally sustain its existence in the
case of an infinite regress of simultaneous movers or causers.
The Third Way begins with the unexceptionable contingent existential fact that there now
exists at least one contingent being. Can some version of the PSR be employed so as to
deduce that there exists a necessary being that causes the existence of this contingent
being? A contingent being has the possibility of not being, and thus, given an infinite
number of times, either through an infinitely extended past or a past time interval that is
comprised of an infinity of moments of time, this possibility will be realized at some past
time. Each moment is like a roll of the dice, an opportunity for this possibility to be
realized. The PSR tells us that something cannot come out of nothing, so there has to be a
cause of this being's coming into existence at this past time. Therefore, something had to