Aquinas' Third Way: Why Argue For Only One Necessary Entity?

Question

I came across this description of Aquinas' third way:

Third, he argues that if there were no eternal, necessary, and immortal being, if everything had a possibility of not being, of ceasing to be, then eventually this possibility of ceasing to be would be realized for everything. In other words, if everything could die, then, given infinite time, everything would eventually die. But in that case nothing could start up again. We would have universal death, for a being that has ceased to exist cannot cause itself or anything else to begin to exist again. And if there is no God, then there must have been infinite time, the universe must have been here always, with no beginning, no first cause. But this universal death has not happened; things do exist! Therefore there must be a necessary being that cannot not be, cannot possibly cease to be. That is a description of God.

source

Two cases are considered here. If "all things are contingent" then all things would eventually cease to exist; but they have not, therefore this case is false. If "all things are contingent except one" then because things still exist that thing must be the first cause (god).

At least two more cases suggest themselves: "more than one (or many) things are not contingent" and "nothing is contingent."

Why are these other cases disregarded, and what conclusions could be reached by considering them?

Answer 1

I don't know about the first case, but the "nothing is contingent" case is excluded by Aquinas's own assumption that we simply encounter contingent things in our everyday experience. Here is a translation of an excerpt from the original text:

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be.

Answer 2

Having talked about arguments such as this one with a professor of mine, I will add to Ram Tobolski's answer only that Aquinas seems to have limited the number of necessary beings to one for the sake of simplicity. You might argue that there ought to be more than one necessary being, but no such arguments were forthcoming, and it seemed to suit Aquinas's purposes just fine to prove that there had to be at least one.

As far as I know, nothing radically counterintuitive falls out from the idea that there may be more than one necessary being rather than only one, but, as was already pointed out, the idea that all beings are necessary wildly opposes experience (and, for the record, our current formulations of physics).

Answer 3

Aquinas's third way falls down because he is observing time and the universe from the inside. he postulates a universe that must be contingent on an infinite, uncaused, prime-mover or it would have ceased to existed under the pressure of time and entropy.

This is pretty much in line with current physical models that suggest the universe will eventually experience a heat-death. Aquinas simply couldn't concieve of the timescales involved. he thought in terms of hundreds and thousands instead of trillions of years. He didn't account for the possibility that the universe he was describing was non-contingent and in the process of dying.