One of the most fascinating puzzles about the nature of time is the question of whether it does or does not have a beginning. It's an issue that has wandered through Western thought for millennia, on the border between philosophy and theology, and no end seems to be in sight.

Possibly the best-known, and certainly the most consequential, intervention in the long conversation is Kant's cunning argument in The Critique of Pure Reason (1781). Here he demonstrates to his own satisfaction that time cannot be something in the world out there, a property of things in themselves; on the contrary, he says, it belongs to the perceiving subject. Time is one lens of the pair of spectacles (the other being space) through which that which is “in-itself” is refracted as it enters into the phenomenal world of experience. (For those who don't feel up to reading the original, Robin Le Poidevin's discussion in his brilliant Travels in Four Dimensions [2003] is an ideal starting-point.)

Kant's argument revolves around the question of whether or not the world has a beginning in time; or (a slither we must watch) whether or not time itself has a beginning. He shows that we can prove both that the world must have and that it can't have a beginning in time, so there must be something wrong with the very idea of time being something in itself.