I’ve been rethinking this post, where I suggested part of the reason the arguments of people like Thomas Aquinas and Samuel Clarke don’t sound convincing today is that they use assumptions which used to be widely accepted, but aren’t anymore. I offered up some caveats about that suggestion at the time, but now I have even more doubts about it.
What prompted this is Leah Libresco’s complaint that the example from Aquinas I used in the previous post only sounds unappealing to modern ears because of Aquinas’ jargon, and if you explained the jargon it would be more appealing. So I went looking for a way to make my original point with a relatively jargon-free example, and found Aquinas’ arguments for the impossibility of an infinite regress of causes (important, because that plays a role in three of his Five Ways):
 Furthermore, that it is impossible for the abovementioned infinites to be moved in a finite time Aristotle proves as follows. The mover and the thing moved must exist simultaneously. This Aristotle proves by induction in the various species of motion. But bodies cannot be simultaneous except through continuity or contiguity. Now, since, as has been proved, all the aforementioned movers and. things moved are bodies, they must constitute by continuity or contiguity a sort of single mobile. In this way, one infinite is moved in a finite time. This is impossible, as is proved in the Physics [VII, 1].
 The second argument proving the same conclusion is the following. In an ordered series of movers and things moved (this is a series in which one is moved by another according to an order), it is necessarily the fact that, when the first mover is removed or ceases to move, no other mover will move or be moved. For the first mover is the cause of motion for all the others. But, if there are movers and things moved following an order to infinity, there will be no first mover, but all would be as intermediate movers. Therefore, none of the others will be able to be moved, and thus nothing in the world will be moved.
 The third proof comes to the same conclusion, except that, by beginning with the superior, it has a reversed order. It is as follows. That which moves as an instrumental cause cannot move unless there be a principal moving cause. But, if we proceed to infinity among movers and things moved, all movers will be as instrumental causes, because they will be moved movers and there will be nothing as a principal mover. Therefore, nothing will be moved.
Now, there are a couple little things that might need explaining here. It’s often said that the word translated here as “move” should be translated “change,” and it’s worth noting that in the first argument, Aquinas appears to assume causes that are simultaneous with their effects. Even with that in mind, though, these arguments look pretty bad.
In the case of the first argument, sure it may seem impossible for one thing to undergo an infinite amount of change in a finite amount of time, but I don’t see any reason why that would mean that an infinite number of things can’t all change simultaneously.
In the case of the second argument, I don’t see how the fact that taking a way the first cause in a finite chain proves that a first cause is in any way special. After all, remove a cause somewhere in the middle of a causal chain, and that also eliminates anything causally downstream of it. That gives you the analogous fact about an infinitely long causal chain, not that the infinite chain would need a first cause too.
And the third argument is just a variation on the second one, and the problem is basically the same. I just don’t see any reason to accept the first premise of the argument.
Now, the broad point here is that while these arguments are supposedly derived from Aristotle, there doesn’t seem to be some secret Aristotelian assumption that would make them work. They’re just plain old bad arguments. I feel comfortable saying this, because respected living philosophers often give arguments that just stink, and being a contemporary of those philosophers I’m confident that the issue isn’t some peculiarly 21st century assumption.
In fact, Aquinas’ arguments feel very similar in their style of badness to William Lane Craig’s bad arguments against the possibility of an infinite past that go into Kalam. Or anyways, that’s true aside from the fact that set theory didn’t exist in Aquinas’ day and therefore he couldn’t get into arguments over its significance.