In mathematics or logic, an unprovable rule or first principle accepted as true because it is self-evident or particularly useful (e.g., "Nothing can both be and not be at the same time and in the same respect").
The term is often used interchangeably with postulate , though the latter term is sometimes reserved for mathematical applications (such as the postulates of Euclidean geometry ). It should be contrasted with a theorem , which requires a rigorous proof.