  As Aristotle sets them out, these problems take the form of puzzles, or aporiai, regarding whether and if so how time exists (Phys. 218a8–30). If we say that time is the totality of the past, present and future, we immediately find someone objecting that time exists but that the past and future do not. According to the objector, only the present exists. If we retort then that time is what did exist, what exists at present and what will exist, then we notice first that our account is insufficient: after all, there are many things which did, do, or will exist, but these are things that are in time and so not the same as time itself. We further see that our account already threatens circularity, since to say that something did or will exist seems only to say that it existed at an earlier time or will come to exist at a later time. Then again we find someone objecting to our account that even the notion of the present is troubling. After all, either the present is constantly changing or it remains forever the same. If it remains forever the same, then the current present is the same as the present of 10,000 years ago; yet that is absurd. If it is constantly changing, then no two presents are the same, in which case a past present must have come into and out of existence before the present present. When? Either it went out of existence even as it came into existence, which seems odd to say the least, or it went out of existence at some instant after it came into existence, in which case, again, two presents must have existed at the same instant. Now, Aristotle does not endorse the claims set out in stating these sorts of aporiai; in fact, very often he cannot, because some aporiai qualify as aporiai just because they comprise individually plausible arguments generating incompatible conclusions.  They thus serve as springboards to deeper, more demanding analysis. 