In control theory, a causal system (also known as a physical or nonanticipative system) is a system where the output depends on past andcurrent inputs but not future inputs—i.e., the output depends on only the input for values of . Classically, nature or physical reality has been considered to be a causal system. Physics involving special relativity or general relativity require more careful definitions of causality, as described elaborately in Causality (physics).