Psychology
You don't run because you're afraid — you feel afraid because you're running
William James and Carl Lange independently proposed, in the 1880s, that emotion isn't what causes a bodily reaction — it's the reverse. See a bear, your body reacts first (racing heart, sprinting legs), and the brain's interpretation of that reaction is the fear itself. It's one of psychology's oldest theories of emotion, and still shapes research into how body and feeling interact.