 The central motivation for epiphenomenalism lies in the premise that whenever there is a sufficient cause of a physical event, there is a sufficient physical cause of that event. If a mental event is something other than a physical event, then for it to make any causal contribution of its own in the physical world would require a violation of physical law. Descartes’ (1649) interactionist model proposed that nonphysical events could cause small changes in the shape of the pineal gland. But such nonphysical effects, however slight, would mean that the physical account of motion is false — for that account says that there will be no such change of shape unless there is a physical force that causes it.