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Analytic/synthetic distinction
The analytic/synthetic distinction divides meaningful propositions into analytic statements, which are true by virtue of meaning alone (e.g. 'all bachelors are unmarried'), and synthetic statements, which are true or false by virtue of how the world is (e.g. 'the cat is on the mat'). Quine challenged and Kripke complicated this division.
Logical positivism
Logical positivism is a philosophical movement originating with the Vienna Circle, holding that only statements verifiable through empirical observation or true by definition are meaningful. All other claims, including metaphysics, theology, and normative ethics, are considered literally cognitively meaningless.
Metaphysics
Metaphysics is the branch of philosophy concerned with the fundamental nature of reality, including questions about existence, substance, causation, and the relationship between mind and body. Ayer argues that metaphysical statements are cognitively meaningless because they are neither analytic nor empirically verifiable.
Verification principle
The verification principle is the criterion of meaning proposed by logical positivists stating that a non-tautological statement is meaningful only if empirical observations could count for or against it, rendering metaphysical and theological claims devoid of propositional content.