Principle of bivalence
This is not to be confused with the law of excluded middle and the law of noncontradiction. See bivalence and related laws for a summary of the differences.
In classical logic, the principle of bivalence is equivalent to the result that there are no propositions that are neither true nor false. A proposition P that is neither true nor false is undecidable. In intuitionistic logic, sometimes the truth-value of a proposition P cannot be determined (i.e. P cannot be proved nor disproved). In such a case, P simply does not have a truth-value. Other logics, e.g. multi-valued logic, may assign P an indeterminate truth-value.