Theory that every proposition is either true or false.
Possible objections are of two kinds.
First, can we decide what counts as a proposition in the relevant sense?
Second, might not the principle fail for some presumably genuine propositions; for example, 'Jones was brave' (where Jones died peacefully after a life entirely devoid of danger)?
The law of bivalence is not necessarily the same as that of excluded middle.
MAE Dummett, 'Truth', Proceedings of the Aristotelian Society (1958-59)