Saint Augustine famously remarked that if someone asked to meet him at a certain time, he would have no problem, but if asked what time is, he would be stumped. Part of the puzzle about the nature of time arises from the fact that subjective experience of time is so various: it can pass in a flash for one of a pair of people watching and enjoying a film, yet drag painfully for the other because the film bores him. If time were an objective thing, how could perceptions of it vary so dramatically?
Differences in perceptions of time make it necessary for us to choose some publicly observable feature—the apparent passage of the sun across the sky, the moon's phases, the radioactive decay of the caesium-123 isotope—as a way of measuring time's passage independently of subjectivities, and indexing those subjectivities to each other.
As one would expect, there has never been a time when philosophers were not interested in time. Historically they are divided between those who thought that time is objective, ticking away regularly independently of whether anything is happening in the universe, and those who thought that time only exists if there is change, so that if nothing is changing anywhere, no time passes. Plato and Newton fall into the first "absolutist" camp, Aristotle and Leibniz into the second "relationist" camp.
As one would also expect, the problem of time has prompted some philosophers to deny that there is such a thing at all. The most famous argument to this conclusion was offered by the handsomely named John McTaggart Ellis McTaggart in 1908. He pointed out that we have two time terminologies; we talk of past, present and future (the "A-series"), and we talk of something being earlier than, simultaneous with, or later than, something else (the "B-series"). He argued that the B-series makes no sense without the A-series because it treats all events as fixed in time, but equally the A-series makes no sense either, because whereas no event can logically possess more than one of the A-properties (of being past, present or future), all events always possess all three because what is present is future from a past perspective and past from a future perspective—and so on for all combinations. It will not do to object that a given event is either past, present or future specifically from the perspective of some other time, says McTaggart, for this simply introduces an unstoppable regress of contradictory predications of A-terms to the perspectives themselves. Therefore, he claimed, time is an illusion.
McTaggart's argument generated a philosophical debate that still continues. Proffered solutions could be discussed another time, if one exists.
Send your philosophical queries and dilemmas to AC Grayling atquestion@prospect-magazine.co.uk
