Asking what time is feels almost too basic to answer, which is exactly why it is so hard. We are so immersed in time that we struggle to get any distance on it. Philosophy makes progress by turning the vague question into sharper ones — Is time a thing, or a relation? Is it the same for everyone? Does its structure include a real 'present'? — and the history of those questions is a story of brilliant, opposed answers.
The classic dispute is between Newton and Leibniz. Newton held that time is absolute: a real, uniform framework that flows evenly and on its own, the same throughout the universe, whether or not anything happens. On this view, time is a kind of container within which all events occur. Leibniz rejected the container entirely. For him, time is relational — it is nothing but the order of events, the network of before, after, and at-the-same-time. Remove the events and you remove time; there is no empty time ticking away in the background. Their disagreement, argued in a famous correspondence, still frames the debate.
A different and equally influential analysis came from J. M. E. McTaggart, who distinguished two ways of ordering events in time. The A-series arranges them as past, present, and future — a tensed ordering that constantly changes, since what is future becomes present and then past. The B-series arranges them only as earlier-than and later-than — a tenseless ordering that never changes, since if one event is earlier than another it always was and always will be. This distinction became the backbone of modern philosophy of time, because nearly every later debate turns on which series captures time's true nature.
The physics twistTime as a dimension.
Modern physics reshaped the question in a way neither Newton nor Leibniz anticipated. Einstein's relativity treats time not as a separate universal backdrop but as a fourth dimension interwoven with the three of space into a single four-dimensional spacetime. Strikingly, it implies that time is not the same for everyone: clocks moving relative to one another tick at different rates, and observers can even disagree about whether two distant events happened at the same time. The comfortable idea of a single, universal 'now' loses its footing.
So what is time, in the end? The honest answer is that it depends on which layer you are asking about, and that no account has swept the field. Physically, time is a dimension of spacetime, real but observer-relative. Metaphysically, the Newton-Leibniz and A-series/B-series debates remain live, because physics tells us how time behaves without fully telling us what it is or whether the passing present is real. Those deeper questions — whether time is genuinely real, whether it flows, and why it has a direction — each get their own page in this wing, and each depends on the groundwork laid here.