In a ‘billiard-ball world’ of Newtonian science, causation was obvious—things had to touch each other in space and a cause always had to precede an effect. But quantum mechanics destroys such notions. What then is causation? Moreover, must causes always be physical? Is ‘mental causation’ a coherent concept? What about ‘top-down causation’? Walter Sinnott-Armstrong is a philosopher specializing in ethics, epistemology, neuroethics, the philosophy of law, and the philosophy of cognitive science. H