principle of conservation See PHILOSOPHY OF SCI -. ENC. principle of contradiction, also called principle of non-contradiction, the principle that a statement and its negation cannot both be true. It can be distinguished from the principle of bivalence, and given certain controversial assumptions, from the principle of excluded middle; but in truth-functional logic all three are regarded as equivalent. Outside of formal logic the principle of (non-)contradiction is best expressed as Aristotle expresses it: ‘Nothing can both be and not be at the same time in the same respect.’ See also LAWS OF THOUGHT, PRINCIPLE OF BIVA- LENC. R.P.