 Determinism is the philosophical theory that all events, including moral choices, are completely determined by previously existing causes. Determinism is at times understood to preclude free will because it entails that humans cannot act otherwise than they do. The theory holds that the universe is utterly rational because complete knowledge of any given situation assures that unerring knowledge of its future is also possible. Some philosophers suggest variants around this basic definition. Deterministic theories throughout the history of philosophy have sprung from diverse and sometimes overlapping motives and considerations. The opposite of determinism is some kind of indeterminism otherwise called non-determinism. Determinism is often contrasted with free will. Determinism often is taken to mean causal determinism, which in physics is known as cause and effect. It is the concept that events within a given paradigm are bound by causality in such a way that any state of an object or event is completely determined by prior states. This meaning can be distinguished from other varieties of determinism mentioned below. Other debates often concern the scope of determined systems, with some maintaining that the entire universe is a single-determinate system and others identifying other more limited-determinate systems or multiverse. Numerous historical debates involve many philosophical positions and varieties of determinism. They include debates concerning determinism and free will, technically denoted as Compatibilistic. Allowing the two to coexist and incompatibilistic, denying their coexistence is a possibility. Determinism should not be confused with self-determination of human actions by reasons, motives, and desires. Determinism rarely requires that perfect prediction be practically possible.