Every proposition is either true or not true.
This is weaker than the law of bivalence (every proposition is true or false), since if there is a third truth value excluded middle can still hold, though bivalence will fail. (However, bivalence is sometimes treated as a version of excluded middle).
For classical logic, excluded middle follows from the law of contradiction. Intuitionist logic accepts the latter but not excluded middle (also see: double negation), for reasons connected with the 'Jones was brave' example (see bivalence).
P T Geach and W F Bednarowski, "The Law of Excluded Middle' (symposium), Proceedings of the Aristotelian Society, supplementary volume (1956)