Philosophy of Space and Time

What it asks

Take everything out of the universe. Is there anything left?

Newton said yes: space and time are real, absolute, and would persist unoccupied. Leibniz said no: space is nothing but the order of things that coexist, and time nothing but the order of things that succeed. Remove the things and you remove the order.

The dispute is three hundred years old, both sides have been reformulated repeatedly, and physics has supplied evidence for each in turn without settling it. The second question is about time alone, and it is worse: whether the passage of time is a real feature of the world or a feature of how we are placed in it.

The recurring problems

Absolute against relational. Leibniz's argument is that a universe shifted six feet to the left would be indistinguishable from this one, so the difference is not a difference. Newton's reply is the spinning bucket: rotation has effects, and there seems to be nothing for it to be rotation relative to.

Does time pass? McTaggart argued in 1908 that the past-present-future series is contradictory and the earlier-later series contains no passage, so time is unreal. Almost nobody accepts the conclusion and the argument has never been cleanly answered.

What exists. Presentism holds that only the present is real, eternalism that all times are equally real, the growing block that the past and present are and the future is not. Relativity is widely thought to favour eternalism, and presentists dispute exactly that.

The arrow. The fundamental laws are almost all time-symmetric. Nothing in them says why eggs break and do not unbreak, and the standard answer, that entropy was low at the beginning, explains the arrow by positing a fact nobody can explain.

How it developed

The Leibniz-Clarke correspondence of 1715 and 1716 is the founding document, with Clarke arguing Newton's case and Leibniz his own. It ended when Leibniz died, which is the usual way such disputes end.

Kant intervened by making space and time neither substances nor relations but forms the mind imposes on experience - a third option that removed the question from physics entirely and is still defended in modified forms.

Relativity changed the terms completely. Space and time turned out not to be separable, simultaneity turned out to be relative to a frame, and spacetime turned out to have a geometry affected by what is in it - which is neither Newton's container nor Leibniz's mere order, and both camps claimed vindication.

The current phase is driven by quantum gravity, where several leading approaches suggest that spacetime is not fundamental at all but emerges from something else. If that is right, the original question was badly posed.

The positions within it

Substantivalism and relationism are the standing positions on space. Presentism, eternalism and the growing block divide over time. The A-theory and B-theory, taking their names from McTaggart's two series, divide over whether passage is real. All are in the Schools section.

Where it stands now

The field is now largely done alongside physics rather than about it, and the philosophers doing it read the papers. The interesting questions have moved to the interpretation of quantum gravity and to whether emergence of spacetime is even coherent.

The metaphysics of passage has not moved with it. Whether time flows remains a question physics does not obviously answer and philosophers cannot obviously drop, since the sense that it does is about as strong as any evidence anywhere.