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What is the verifiability principle used by logical positivists?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — metaphysics Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle (c. 325 bce); in the collection of the Museo Nazionale Romano, Rome.

The verifiability principle is a foundational doctrine of logical positivism that asserts a statement is cognitively meaningful only if it is either empirically verifiable or tautological (true by definition).12

According to this criterion, any statement that cannot be tested through observation or experiment—such as those concerning metaphysics, ethics, aesthetics, or religion—is considered assertorically meaningless. While these statements may influence emotions or conduct, they do not impart knowledge because they cannot be proven true or false through experience.312

The principle evolved significantly over time. Early formulations required that a statement be logically entailed by a finite set of observation reports. Later, as this proved too restrictive for scientific laws, it was refined into a more flexible requirement of "confirmability" or "testability," where statements are considered meaningful if they are supported or rendered probable by evidence.14

A major criticism of the principle is that it is not itself an empirical proposition, leading some to argue that it is either self-refuting or merely an arbitrary recommendation for scientific inquiry.1