a philosophical doctrine fundamental to the school of Logical Positivism holding that a statement is meaningful only if it is either empirically verifiable or else tautological (i.e., such that its truth arises entirely from the meanings of its terms). Thus, the principle discards as meaningless the metaphysical statements of traditional philosophy as well as other kinds of statements—such as ethical, aesthetic, or religious principles—asserted as true but neither tautological nor known from experience. Such statements may have meaning in the sense of being able to influence feelings, beliefs, or conduct but not in the sense of being true or false and hence of imparting knowledge. According to the principle, then, a nontautological statement has meaning only if some set of observable conditions is relevant to determining its truth or falsity; so stated, it reflects the view that the meaning of a statement is the set of conditions under which it would be true.

Disagreement among Positivists has arisen over the kinds of conditions that qualify as observable for purposes of verification and the exact nature of their relevance to the truth or falsity of statements. While Hans Reichenbach has maintained that verifying observations must be physically possible or compatible with the known laws of science, it has been more widely held that they need only be logically possible or conceivable in a noncontradictory way. Early exponents of the view that observation reports provide an indubitably certain foundation for knowledge held that verifiability requires that a statement be logically entailed by some finite set of observation reports. Later Positivists, having abandoned this view, have required of a verifiable statement only that it be made evident or supported or rendered probable by the relevant set of observations.

The principal criticism of the verifiability principle has been that, because it is not an empirical proposition, it is itself on its own terms either meaningless or else tautologically true as an arbitrary definition of meaningfulness. In response, it has been argued both that the principle is indeed a tautology, though a nonarbitrary one in that it reflects actual usage and that it is strictly meaningless but to be taken as a recommendation for the conduct of scientific inquiry.