Chapter 5: The Verification Principle

The earliest and most austere version of the verification principle held that a (synthetic) statement is meaningful if and only if it is conclusively verifiable by sensory experience. That is, there must be a finite set of observations that would establish the truth of the statement beyond any possibility of doubt.

This version immediately faced devastating objections:

• •Universal statements: “All copper conducts electricity” is a universal generalization that cannot be conclusively verified by any finite set of observations (there are infinitely many pieces of copper). Yet universal laws are the pride of science. Strong verification would render them meaningless.
• •Statements about the past: “Caesar crossed the Rubicon” cannot be conclusively verified by present or future observations. Strong verification would make historical statements meaningless.
• •Statements about other minds: “She is in pain” cannot be conclusively verified by any observation of behavior (behavior could be faked). Strong verification threatens to make all ascriptions of mental states to others meaningless.
• •Theoretical statements: “Electrons have a charge of 1.6 × 10⁻¹⁹ coulombs” cannot be directly observed. Strong verification would make much of theoretical physics meaningless.

These objections made it clear that strong verification was far too restrictive. It eliminated not only metaphysics but most of science itself. The positivists quickly retreated to weaker formulations.

The retreat from strong to weak verification was swift. Instead of requiring conclusive verification, the weakened principle required only that a statement be confirmable in principle — that some possible observation would count as evidence for or against it.

Weak verification successfully accommodated universal laws and theoretical statements: the observation of a copper wire conducting electricity does confirm (though not conclusively verify) the universal generalization “All copper conducts electricity.” But it introduced new problems:

• •Too permissive: Alonzo Church showed (1949) that certain formulations of weak verification allow virtually any statement to count as meaningful. If S is any statement whatever, then “If O then S” is confirmed by observing O. So even the most outlandish metaphysical claims can be rendered “meaningful” by suitable logical manipulation.
• •Vagueness of “relevant”:What does it mean for an observation to be “relevant” to a statement? Without a precise account of relevance, the principle is too vague to do the work required of it.

A.J. Ayer (1910–1989) brought logical positivism to the English-speaking world with Language, Truth and Logic (1936), written when he was just 25. The book is a brilliantly lucid and provocatively combative exposition of the positivist programme. Ayer distinguished between two kinds of meaningful statement:

Ayer proposed two criteria:

“We say that a sentence is factually significant to any given person, if, and only if, he knows how to verify the proposition which it purports to express — that is, if he knows what observations would lead him, under certain conditions, to accept the proposition as being true, or reject it as being false.”— A.J. Ayer, Language, Truth and Logic (1936), p. 35

Ayer’s formulations were subjected to devastating technical criticisms. Alonzo Church and others showed that indirect verifiability was either too restrictive (excluding legitimate scientific statements) or too permissive (admitting metaphysical statements). Ayer himself conceded in the 1946 introduction to the second edition that he had not found a satisfactory formulation, and he continued to struggle with the problem for the rest of his career.

The most famous objection to the verification principle is that it is self-refuting. Consider the verification principle itself:

Is this statement itself analytic or synthetic? It does not seem to be analytically true — it is not true by virtue of the meanings of its terms (one can understand the words without accepting the principle). If it is synthetic, then by its own criterion it must be empirically verifiable. But what observation could verify or falsify it? No empirical test could establish that meaningfulness requires verifiability. So the verification principle, by its own criterion, is meaningless.

“The positivists’ attempt to eliminate metaphysics by means of the verification principle was comparable to the attempt of the Baron Münchhausen to pull himself out of a swamp by his own bootstraps.”— (A common philosophical joke, often attributed to various philosophers)

The positivists offered several responses:

• •It is analytic: Some positivists argued that the principle is a definition of “meaningful” and is therefore analytic. But this is question-begging: why should we accept this definition?
• •It is a proposal: Carnap suggested that the principle is not a truth-claim but a proposal — a recommendation about how to use the word “meaningful.” Proposals are neither true nor false, so the self-refutation objection does not apply. But this weakens the principle considerably: if it is merely a proposal, others are free to reject it.
• •It is a higher-order principle: The principle applies to first-order statements about the world, not to second-order statements about meaning. But this seems ad hoc: why should the principle exempt itself?

None of these responses has been widely accepted. The self-refutation objection remains one of the most effective criticisms of the verification principle, and it contributed significantly to the decline of logical positivism.

Universal statements — the laws of science — posed a persistent problem for verificationism. “All metals expand when heated” ranges over infinitely many instances, past, present, and future. No finite set of observations can verify it. Yet such statements are the core of scientific knowledge.

Schlick tried to resolve this by treating universal laws not as genuine statements but as rules of inference — instructions for generating predictions from particular observations. “All metals expand when heated” is not a proposition that is true or false but a schema that licenses the inference from “This is a metal being heated” to “This will expand.” But this seemed to rob science of its content: surely Newton’s law of gravitation says something about the world, not just about our inferential habits.

Carnap attempted a more sophisticated solution through his theory of degree of confirmation. Universal statements can be increasingly confirmed (but never conclusively verified) by their instances. This approach anticipates Bayesian confirmation theory (Chapter 19) and represents one of the most lasting contributions of the positivist programme.

“The laws of nature, so-called, are not truth-functions of observation sentences... they have a different logical character; they are rules for the formation of observation sentences, and thus have the character of prescriptions.”— Moritz Schlick, “Meaning and Verification” (1936)

Rudolf Carnap’s response to the mounting difficulties of verificationism was to shift from a material to a formal mode of discourse. In The Logical Syntax of Language (1934), Carnap argued that philosophical questions are not about the world but about language. The question “Are there numbers?” is not a metaphysical question about the furniture of the universe but a question about whether to adopt a linguistic framework that includes number-talk.

Carnap’s principle of tolerance represented a significant liberalization:

“In logic, there are no morals. Everyone is at liberty to build up his own logic, i.e. his own form of language, as he wishes. All that is required of him is that, if he wishes to discuss it, he must state his methods clearly, and give syntactical rules instead of philosophical arguments.”— Rudolf Carnap, The Logical Syntax of Language (1934), §17

This was a remarkable evolution. The early positivists had sought to eliminate metaphysics by showing it to be meaningless. Carnap was now suggesting that the choice between linguistic frameworks (including ones that countenance abstract entities) is a practical matter, not a theoretical one. This distinction between internal questions(questions within a framework, e.g., “Is 7 a prime number?”) and external questions (questions about whether to adopt a framework, e.g., “Do numbers exist?”) became central to Carnap’s later philosophy.

The principle of tolerance effectively conceded that the verification principle cannot serve as a universal criterion of meaningfulness. Different linguistic frameworks may have different criteria, and the choice between frameworks is pragmatic rather than factual. This move saved Carnap from the self-refutation objection but at the cost of abandoning the positivists’ original ambition of a single criterion of meaning.

The history of the verification principle is a history of successive liberalizations, each designed to accommodate objections but each introducing new problems:

Carl Hempel’s influential papers “Problems and Changes in the Empiricist Criterion of Meaning” (1950) and “The Empiricist Criterion of Meaning” (1951) systematically documented the failures of every proposed version. Hempel, himself a former member of the Berlin Circle (a sister group to the Vienna Circle), concluded that the verification principle had proven untenable.

“It seems increasingly clear that the concept of cognitive significance is best characterized, not by an all-or-nothing dichotomy of meaningful and meaningless, but by a graded concept of significance.”— Carl Hempel, “Empiricist Criteria of Cognitive Significance” (1965)

By the 1960s, the verification principle had been effectively abandoned even by philosophers sympathetic to empiricism. Quine’s “Two Dogmas of Empiricism” (1951) delivered the final blow, arguing that the very distinction between analytic and synthetic statements — on which the verification principle depended — was untenable (see Chapter 6).

The verification principle is almost universally regarded as a failure. But its failure was not barren. Several important insights emerged from the debate:

• •Empirical content matters: Even if the verification principle is too strict, the underlying intuition — that meaningful empirical claims must have some connection to possible experience — remains compelling. A theory that makes no observable difference is hard to take seriously.
• •Meaning is complex: The failure of the verification principle showed that meaning cannot be reduced to a simple relationship between statements and observations. This opened the way for more sophisticated theories of meaning (Quine’s holism, Davidson’s truth-conditional semantics, Brandom’s inferentialism).
• •The rehabilitation of metaphysics: The failure of verificationism removed the most powerful argument against metaphysics. Contemporary analytic metaphysics — from possible worlds to grounding to mereology — is partly a consequence of this failure.
• •Confirmation theory: The attempt to make the verification principle precise led directly to the development of confirmation theory — the formal study of how evidence supports hypotheses — which remains a thriving area of philosophy of science.

“The death of logical positivism is one of the best things that ever happened to philosophy. But the spirit of empiricism it embodied — the commitment to taking experience seriously as a source and test of knowledge — is as vital as ever.”— (A common sentiment among contemporary empiricists)