McTaggart argues that the A series (a series of events being past, present, and future) is contradictory because an event cannot be past, present, and future, yet any event in the A series will be all three.
His response to a possible refutation (from his essay, "The Unreality of Time"):
If we avoid the incompatibility of the three characteristics by asserting that M is present, has been future, and will be past, we are constructing a second A series, within which the first falls, in the same way in which events fall within the first. It may be doubted whether any intelligible meaning can be given to the asser- tion that time is in time. But, in any case, the second A series will suffer from the same difficulty as the first, which can only be removed by placing it inside a third A series. The same principle will place the third inside a fourth, and so on without end. You can never get rid of the contradic- tion, for, by the act of removing it from what is to be explained, you produce it over again in the explanation. And so the explanation is invalid.
I don't understand where the infinite regress comes from. He says that we are constructing a second A-series, but how so? I don't understand what there is to say after saying "M is present, has been future, and will be past."
Thanks in advance.