It is over a century since J. M. E. McTaggart's “The Unreality of Time” was published and its argument still looms large. No one, it seems, can write about the philosophy of time without engaging with it. And yet its first step incorporates an assumption that, fatally for McTaggart's position, is mistaken. But since, as is often the case in philosophy, his mistake is illuminating, it is worth teasing out. McTaggart's attack on time has two steps. The first is to show that tensed, or what he calls “A-series”, time, which involves the idea that events begin in the future, become present and end up in the past, is unreal because contradictory. The second step is to demonstrate that the apparent reality of tenseless or “B-series” time (“earlier” and “later”) depends on tensed time. Time itself is therefore unreal.

If we believe in the reality of tensed or A-series time, McTaggart argues, we must hold that events that are future are also present and past. But being future and present and past are incompatible states: the same thing cannot be “not-yet”, “now” and “no-longer”.