David at "Miscellaneous musings on this and that" discusses Aquinas's First way here and here (hat-tip: Dangerous Idea). Just a few points:
* The Summa was written as an introductory theology textbook; but the students who were expected to use it would all have studied Aristotle for several years. The First Way is manifestior, i.e., more obvious, for such a person, particularly given the obviousness of change to the senses.
* On the inertia objection: I have discussed the matter here. Briefly: (1) Since motus is not confined to locomotion (which is just motus or motion in place), this is not actually an issue for the argument at all, unless analogous objections could be made to work for other cases of change. All the argument needs is one (this is a problem with the argument in David's second post; it proceeds on the false assumption that the argument only works if it covers every case of change; I think with Aquinas's interpretation it does in fact work for every form of change that falls under the concept of motus, but this is more than the argument strictly needs to reach its conclusion). (2) But as Weisheipl pointed out, Newton's own formulation doesn't actually conflict with the principle; (3) and, as I pointed out, Whewell's nineteenth-century analysis of the relation between the inertia principle and causal principles, which is the best that has been done, has the result (allowing for some difference in philosophical idiom) of making the inertia principle a corollary of the sort of causal principle Aquinas uses. (4) In any case, Aquinas's interpretation of the 'whatever is moved is moved by another' principle, as it is unfolded elsewhere in Aquinas's work, doesn't have any problem with persistence in a state of rest or motion: the mover in such a case is (proximately) the nature of the moving object and (remotely) the generator of the nature (or, alternatively, the remover of impediment to motion). In short, it is actually doubtful that there is any problem here. No one has actually done a non-question-begging analysis to show that there is any problem at all; and, as I noted, Whewell's analysis, which has to my knowledge no competitors, suggests exactly the contrary.
But I'm glad it's being discussed. David's right that the First Way is probably not particularly useful for apologetic purposes; but that's a rhetorical limit to the argument, not something that is problematic for its demonstrations. Demonstrations generally are not particularly useful for apologetics, because, however, good the demonstration may be, the skeptic will always be tempted simply to deny the premises in order to avoid the conclusion. Demonstrations get you from the premises (whatever they may be) to the conclusions; but apologetics primarily needs to get you to the point where you begin to recognize the plausibility of the premises (whatever they may be).
