Attacking the A-series, McTaggart argues that any event in the A-series is past, present, "and" future, which is contradictory in that each of those properties excludes the other two. McTaggart admits that the contradictory nature of the A-series may not be obvious, because it would appear that events never are simultaneously future, present, and past, but only successively so. However, there is a contradiction, he insists, because any attempt to explain why they are future, present, and past, "at different times" is (i) "circular" because we would need to describe the successive order of those "different times" again by invoking the determinations of being future, present "or" past, and (ii) this in turn will inevitably lead to a "vicious infinite regress". The vicious infinite regress arises, because to explain why the second appeal to future, present, and past, doesn't lead again to the same difficulty all over, we need to explain that they in turn apply successively and thus we must again explain that succession by appeal to future, present, and past, and there is no end to such an explanation. It is the validity of the argument in favour of a vicious infinite regress that has received the most attention in 20th Century philosophy of time.