I have been grading take-home quizzes on the history of philosophy for my intro course. One of the questions was "In Thomas Aquinas's First Way, what do you think is the weakest premise (the one that would require the most work to defend), and why?" The point of it was primarily to see if they had learned what 'the First Way' meant, but having phrased it in this roundabout way this time around, I think I got more interesting answers than I have previously (which asked for a summary of it). To some extent the answers aren't wholly surprising, and no great weight can be put on them, given that students only had had one class on the argument, which was primarily geared to simply providing a tour of the medieval scholastic approach to philosophy, and inovlved only a very light summarizing of the background for the argument. So you get some standard confusions and incoherent arguments. But, of course, that's about the level at which most people approach it, so it provided an interesting sample of the range of immediate responses people might have. That's certainly useful. (And I have to say that in some cases my students make more perceptive responses on the basis of one class than I've sometimes heard from professional philosophers; a sign, I think, of the occasional laziness of the latter.) Here are the answers from the quizzes that were turned in on time which had answers for this particular question. I have paraphrased and abbreviated pretty much all of them.
* "This cannot go on to infinity because there would be no first mover." This is begging the question; for all we know, it can go on to infinity.
* "This everyone understands to be God." Aquinas would need to be able to prove the existence of God for this to be true. The belief that God exists is not evidence that God exists.
* The idea that it is impossible for a thing to be both mover and moved, because the argument over the first mover or cause, as God, does not admit of proof.
* "Whatever is moved must be moved by another." We move and we are moved by our own will. Plenty of things move themselves.
* "This everyone understands to be God." Not everyone believes in God, and some people don't believe God is a mover.
* That the motion of the whole world and each motion within it is caused by the motion of the heavens, because he believed in geocentrism. [This may seem to come a bit out of nowhere, but it's not really the student's fault. The nice thing about take-home quizzes is that students can cite the sources they get their information from, and there was a source here.]
* The idea that an infinite number of movers is impossible. The law of conservation of energy states that energy is neither created nor destroyed, so it is possible for there to be an infinite number of movers.
* That nothing can be in actuality and potentiality at the same time. Something can be on fire and still have the potential to burn.
* That there must be a first mover. It seems strange to me that God just came to be, and that there was nothing that set him in motion.
* The one that attributes first motion to God. If everything that happens has to have a first cause, why does the first cause have to be God? If God is the cause of everything and everything requires a cause, what is God's cause?