Principle that to be meaningful a sentence or proposition must be either verifiable by means of the five senses or a tautology of logic.
The verifiability might be required in practice or (more usually) in principle, and might need to be conclusive (strong verifiability) or could be merely partial (weak verifiability).
Mathematical sentences are treated as tautologies. All others (of metaphysics, ethics, religion, and so on) have meaning, if at all, only in some secondary way (see also speech act theories).
Sometimes the principle says that the meaning is the method of verification, and then the principle can be called the verifiability (or verification) theory of meaning, though this title sometimes refers simply to the claim that the principle, however formulated, should be accepted.
Among objections to the principle are that it cannot apply to itself, and is in danger of excluding too much (propositions in science and history, and so on); but a derivative of it has recently appeared as anti-realism.
Also see: logical positivism
A J Ayer, Language, Truth and Logic (1936); 2nd edn with important new 'Introduction', 1946