Free will
The philosophical question of whether human beings are genuinely capable of making choices that are not fully determined by prior causes. The debate has three main positions: hard determinism holds that all events, including human decisions, are causally necessitated by prior states of the world; libertarian free will holds that humans have genuine causal power over their choices that escapes physical determinism; and compatibilism holds that free will, properly understood, is compatible with determinism. The rise of the many-worlds interpretation of quantum mechanics adds a new wrinkle: if every quantum event causes the universe to branch, then in some sense "everything happens" — but this seems to undermine the significance of individual choice rather than restore it, since you cannot choose which branch of the multiverse you inhabit.