The thesis that physics is causally closed asserts that:

Every physical effect has a sufficient immediate physical cause, insofar as it has a sufficient immediate cause at all.

If this thesis is true, it distinguishes physics from all other subject domains. The biological realm is not causally closed, for example, because biological effects often have nonbiological causes, as when the impact of a meteorite precipitated the extinction of the dinosaurs. Again, meteorology is not causally closed: The burning of carbon fuels—a nonmeteorological event—is causing global warming. Nor, importantly, is the mental realm causally closed: A mental pain can be caused by sitting on a physical thumbtack, and a train of thought can be interrupted by a loud noise.

Physics, by contrast, does seem to be causally closed. If one considers any physical effect, then there will...