Determinism, philosophical doctrine that holds that every event, mental as well as physical, has a cause, and that, the cause being given, the event invariably follows. This theory thus denies the element of chance or contingency. It is also opposed to indifferentism, or indeterminism, which maintains that, in phenomena of the human will, preceding events do not definitely determine subsequent ones. Because determinism is generally assumed to be true of all events except volition, the doctrine is of greatest importance when applied to ethics. See also Fatalism; Free Will; Predestination; Uncertainty Principle.