Language, Truth and Logic

Written by the British philosopher A. J. Ayer and first published in 1936, Language, Truth and Logic is a seminal work of analytic philosophy that introduced logical positivism to an English-speaking audience. Logical positivism holds that only analytic or empirically testable statements are meaningful. Drawing on the empiricist tradition of George Berkeley and David Hume and the more recent work of Bertrand Russell, Ludwig Wittgenstein, and the Vienna Circle (a group of philosophers and scientists centered in Vienna who advocated a scientific worldview grounded in logic and empirical verification), Ayer advances a single thesis: A statement is literally meaningful only if it is either analytic (true by definition) or empirically verifiable, and any utterance that fails both tests is not false but nonsensical. The second edition, published in 1946, includes an introduction in which Ayer revisits and refines several central arguments.

In the Introduction to the second edition, Ayer acknowledges that the questions the book addresses are less simple than the original text suggests, though he still considers its point of view substantially correct. He introduces "statement" as a technical term for what any indicative sentence expresses, reserving "proposition" for what is expressed by literally meaningful sentences. He distinguishes between "strong" verifiability, which requires conclusive establishment of truth in experience, and "weak" verifiability, which requires only that experience render a proposition probable. He concedes that his original strong sense had no real application, since all empirical propositions remain open to further testing. He refines his criterion by defining "direct" verifiability (statements that are observation-statements or that, conjoined with observation-statements, entail new ones) and "indirect" verifiability (statements that, conjoined with already-verified premises, entail directly verifiable statements). He now regards the principle of verification, his rule for determining when a sentence counts as literally meaningful, not as an empirical hypothesis but as a definition, though he concedes it may need supplementing by detailed analyses of particular metaphysical arguments.

In the Preface to the first edition, Ayer divides all genuine propositions into two classes: those concerning "relations of ideas" (the a priori, or experience-independent, propositions of logic and mathematics, which are analytic tautologies) and those concerning "matters of fact" (empirical hypotheses that can be probable but never certain). He declares that it cannot be significantly asserted that there is a non-empirical world of values, that people have immortal souls, or that there is a transcendent God.

Chapter I, "The Elimination of Metaphysics," introduces the verification criterion, the rule that meaningful factual claims must connect to possible observation. Ayer argues that the metaphysical thesis that philosophy yields knowledge of a transcendent reality must be criticized at the level of the statements themselves. He distinguishes his approach from that of Immanuel Kant, the German philosopher: Kant treated the impossibility of transcendent metaphysics as a fact about the human mind's limitations, whereas Ayer treats it as a consequence of the rules governing meaningful language. A sentence is factually significant only if the person who utters it knows what observations would lead to its acceptance or rejection. Ayer rejects conclusive verifiability as the standard, since general laws cannot be verified by any finite set of observations, and likewise rejects conclusive falsifiability, since one can always question the conditions of an experiment. He settles on a weaker criterion: A statement is genuinely factual if some experiential proposition can be deduced from it, in conjunction with other premises, that is not deducible from those other premises alone. Applying this criterion, he condemns as nonsensical the claim that sense-experience is unreal, disputes over the number of substances, and the metaphysical dimension of the debate between realists (who hold the external world exists independently of perception) and idealists (who tie reality to mind or experience). He traces the origins of metaphysics to linguistic confusion, particularly the tendency to treat grammatical features as evidence of real entities.

In Chapter II, "The Function of Philosophy," Ayer argues that philosophy is wholly critical and analytic rather than speculative. He rejects the idea that philosophy should build deductive systems from first principles, criticizes the French philosopher René Descartes' cogito ergo sum ("I think, therefore I am") as logically invalid, and contends that the problem of induction is fictitious, since any attempt to prove that past experience reliably guides future expectation either commits a logical fallacy or begs the question. He reclaims much of the philosophical tradition as analytic, presenting Berkeley, Hume, and others as analysts of perception and causation rather than metaphysicians. Philosophy's propositions, he concludes, are linguistic in character: they express definitions or the formal consequences of definitions.

Chapter III, "The Nature of Philosophical Analysis," explains the method of definition in use, which translates sentences containing a given symbol into equivalent sentences that do not contain it. Ayer introduces the concept of a "logical construction": To say an entity is a logical construction out of certain elements is to say that all sentences about it can be translated into sentences about those elements. Material things, he argues, are logical constructions out of sense-contents, the immediately given elements of sensory experience. He sketches how a material thing's identity can be analyzed through relations of resemblance and continuity among visual and tactual sense-contents.

Chapter IV, "The A Priori," addresses the central challenge facing empiricism: accounting for the apparent necessity of logic and mathematics. Ayer rejects the British philosopher John Stuart Mill's view that these truths are inductive generalizations, noting that we never allow apparent counterexamples to confute them. Instead, he argues the truths of logic and mathematics are analytic, true solely in virtue of the definitions of their constituent symbols and devoid of factual content. He applies this analysis to geometry, arguing that axioms are definitions and theorems their logical consequences, thereby rejecting Kant's claim that geometry and arithmetic involve synthetic a priori knowledge—synthetic meaning propositions whose truth extends beyond mere definitions to make claims about the world. He addresses the objection, raised by the French mathematician Henri Poincaré, that mathematics cannot amount to mere tautology, arguing that its power to surprise us reflects limitations of human intellect rather than any factual content.

In Chapter V, "Truth and Probability," Ayer argues that "is true" is logically superfluous: to call a proposition true is simply to assert it. He denies that any empirical proposition is absolutely certain, holding that all are hypotheses confirmed or discredited by systems of observation. Hypotheses function as rules governing expectations of future experience.

Chapter VI, "Critique of Ethics and Theology," develops the emotive theory of ethical judgments, the view that ethical statements express feelings rather than state facts. Ayer argues that ethical concepts are pseudo-concepts whose presence in a sentence adds nothing to its factual content. Ethical statements are expressions of feeling, neither true nor false. He addresses an objection from the philosopher G. E. Moore that this theory makes moral argument impossible, responding that apparent value disputes are really disputes about facts, and that when factual agreement is reached but moral disagreement persists, argument is at an end. Turning to theology, he argues that "God exists," if it means more than the occurrence of certain natural regularities, is a metaphysical utterance devoid of literal significance. He distinguishes his position from both atheism and agnosticism, since the sentence "There is a transcendent god" expresses no proposition at all.

Chapter VII, "The Self and the Common World," analyzes the self as a logical construction out of sense-experiences. Two experiences belong to the same self if they contain organic sense-contents, or bodily sensations, that are elements of the same body. Ayer rejects the substantive ego as metaphysical and addresses the charge of solipsism, the philosophical worry that only one's own mind can be known to exist, by arguing that other people must be defined in terms of their empirical manifestations, just as material things are.

The final chapter applies Ayer's framework to philosophy's great disputes. Against rationalists (who maintain that thought is an independent and superior source of knowledge), he reaffirms that a priori propositions are tautologies. He distinguishes his own logical empiricism, an empiricist program combining verificationism with logical analysis, from the Vienna Circle's stricter positivism, whose criterion of conclusive verifiability is self-defeating. Against idealists, he argues that material things can exist unperceived because they are logical constructions whose elements include possible as well as actual sense-contents. Against monists (who hold that reality is fundamentally one unified whole), he exposes as fallacious the assumption that all properties of a thing are defining properties, showing it leads to the absurd conclusion that no synthetic proposition is possible. He concludes by asserting the unity of philosophy with science: Philosophy does not compete with science but clarifies its concepts, and must develop into the logic of science.