The Verification Theory of Meaning, propounded by Logical Positivists (like A.J. Ayer and Rudolf Carnap) of the Vienna Circle, asserted a radical criterion for meaning: a statement is meaningful only if it is either analytically true (a tautology, verifiable by logic or definition, e.g., "All bachelors are unmarried") or empirically verifiable. Its purpose was to distinguish meaningful (scientific, logical) discourse from what they considered meaningless (metaphysical, ethical, aesthetic) statements, which could not be confirmed by experience. This theory aimed to cleanse philosophy of non-empirical speculation.

In this context, a crucial differentiation was made between the "strong" and "weak" senses of "verifiable":

1. Strong Verifiability: A statement is meaningful if its truth can be conclusively established by empirical observation. For instance, "There is a black cat in this room" can be conclusively verified by seeing the cat.

   • Limitation: This strong sense quickly proved too restrictive, as universal scientific laws (e.g., "All metals expand when heated") or historical statements could never be conclusively observed in all possible instances. This would render most scientific laws meaningless.

2. Weak Verifiability: To address this, a "weak" sense was introduced: a statement is meaningful if some empirical observation can make it probable or contribute to its truth or falsity. It does not require conclusive proof.

   • Advantage: This broadened the scope of meaningful statements to include universal scientific laws and historical claims, as some evidence could make them probable.

   • Limitation: Critics argued this weak sense became too permissive, potentially allowing almost any statement (even traditionally metaphysical ones) to be weakly verifiable by some imaginable observation, thus defeating the original purpose of the theory.

Ultimately, the Verification Theory of Meaning, despite its influence, faced significant internal challenges and criticisms, contributing to its eventual decline as a dominant philosophical framework.