H. P. Grice: "Tertium non datur"

TERTIUM NON DATUR -- bivalence Logic A basic principle of classical or standard logic, according to which every statement or proposition must be either true or false. It is closely associated with the law of the excluded middle, but its status is controversial in modern non-standard logic. Many logicians and philosophers claim that some statements or propositions (for example, future contingents, mathematical claims without constructive proofs, or paradoxical, vague, or modal statements) are neither true nor false, but rather have an intermediate truth-value. Modern systems of multivalued logic, partly motivated by such claims and partly developed as important formal investigations in their own right, are truth-valueless or have from three truth-values to an infinite number of truthvalues. Since Dummett, this principle has become the focus of the debate between realism and antirealism. According to anti-realism, the basic position of realism is to hold that a statement must be either true or false, no matter whether we know it. “The principle that every statement is true or false is called the principle of Bivalence.” Kneale and Kneale, The Development of Logic