Debate: What is time?

Presentism

Only the present exists. The past existed and the future will exist, but neither does.

Best argument

This is common sense, and it fits our experience: we remember the past and anticipate the future, but we only ever live in the present. It explains why time seems to flow: what exists keeps changing. It also explains the asymmetry we feel between a past that is fixed and gone and a future that is open.

Strongest objection

Relativity says there is no absolute present. Which events are simultaneous depends on your state of motion. If only the present exists, existence itself becomes relative to the observer, which seems absurd. And if the past does not exist, what makes statements about it true?

The reply

Presentists answer that relativity describes how we measure time, not what exists, and that some versions of physics allow a preferred cosmic frame. Truths about the past can be grounded in present traces or in present-tense facts about what was the case.

Defended by

Augustine of Hippo, A. N. Prior, Dean Zimmerman

Eternalism: the block universe

Past, present and future events all exist equally. 'Now' is like 'here': it marks your location, not a special status.

Best argument

Relativity strongly suggests that there is no universal present, so no objective fact about which slice of reality is the 'real' one. The natural conclusion is that the whole four-dimensional spacetime exists. The passage of time is how a being that remembers in one direction and not the other experiences its path through the block.

Strongest objection

It seems to make change an illusion and our experience of time's passage systematically misleading. If my future already exists, it is hard to see how my choices matter.

The reply

Eternalists answer that change is real in the block: it is simply variation along the time dimension, as a road changes along its length. And choices matter because they are part of what makes the future what it is.

Defended by

Albert Einstein, Hermann Minkowski, J. J. C. Smart

The growing block

The past and present exist, but the future does not yet. Reality grows as new moments come into being.

Best argument

This keeps what is plausible in both views: the past is fixed and real (so statements about it have something to be true of) while the future is open. It also gives time a direction and a real passage: the edge of the block moves.

Strongest objection

If the past exists just as the present does, how do we know we are at the growing edge? People in the past would have the same experiences and the same conviction that they were in the present.

Defended by

C. D. Broad, Michael Tooley

Time is unreal

Time, at the deepest level, does not exist. It is an appearance or an emergent feature of something timeless.

Best argument

J. M. E. McTaggart argued in 1908 that the idea of past, present and future is contradictory, since every event has all three properties, and that without them there is no real time. Some approaches to quantum gravity describe the universe with equations in which time does not appear.

Strongest objection

McTaggart's argument has been widely contested, and physics that lacks a time variable at the deepest level still needs to explain why time appears at ours. Calling it 'unreal' may only mean it is not fundamental.

Defended by

J. M. E. McTaggart, Parmenides, Julian Barbour

Where you stand

Does time really pass?

Choose one to see how people answer.

For the full background, variations and open questions, read the entry: Is time real?.