No human watched the continents move into their present positions.
No astronomer stood beside the neutron stars that collided before the signal from GW170817 began its journey toward Earth.
No scientist observed the first generations of stars switch on.
Yet science reconstructs events like these.
It does so because the past leaves witnesses.
Photons arrive.
Rocks retain chemical changes.
Fossils preserve anatomy.
Gravitational waves cross space.
Detectors register transient events.
Later states of the universe contain physical consequences of earlier ones.
That makes historical science possible.
It does not make historical reconstruction unlimited.
Reconstructing the Past asks where the limit lies.
Its central idea is straightforward:
A past can be recovered only as finely as surviving evidence can distinguish it from alternative pasts.
Imagine a perfectly deterministic physical law.
Give it an initial state and it produces a later state.