principle of verifiability a claim about what meaningfulness is: at its simplest, a sentence is meaningful provided there is a method for verifying it. Therefore, if a sentence has no such method, i.e., if it does not have associated with it a way of telling whether it is conclusively true or conclusively false, then it is meaningless. The purpose for which this verificationist principle was originally introduced was to demarcate sentences that are ‘apt to make a significant statement of fact’ from ‘nonsensical’ or ‘pseudo-‘ sentences. It is part of the emotive theory of content, e.g., that moral discourse is not (literally, cognitively) meaningful, and therefore, not factual. And, with the verifiability principle, the central European logical positivists of the 1920s hoped to strip ‘metaphysical discourse’ of its pretensions of factuality. For them, whether there is a reality external to the mind, as the realists claim, or whether all reality is made up of ‘ideas’ or ‘appearances,’ as idealists claim, is a ‘meaningless pseudo-problem.’
The verifiability principle proved impossible to frame in a form that did not admit all metaphysical sentences as meaningful. (Further, it casts doubt on its own status. How was it to be verified?) So, e.g., in the first edition of Language, Truth and Logic, Ayer proposed that a sentence is verifiable, and consequently meaningful, if some observation sentence can be deduced from it in conjunction with certain other premises, without being deducible from those other premises alone. It follows that any metaphysical sentence M is meaningful since ‘if M, then O’ always is an appropriate premise, where O is an observation sentence. In the preface to the second edition, Ayer offered a more sophisticated account: M is directly verifiable provided it is an observation sentence or it entails, in conjunction with certain observation sentences, some observation sentence that does not follow from them alone. And M is indirectly verifiable provided it entails, in conjunction with certain other premises, some directly verifiable sentence that does not follow from those other premises alone and these additional premises are either analytic or directly verifiable (or are independently indirectly verifiable). The new verifiability principle is then that all and only sentences directly or indirectly verifiable are ‘literally meaningful.’ Unfortunately, Ayer’s emendation admits every nonanalytic sentence. Let M be any metaphysical sentence and O1 and O2 any pair of observation sentences logically independent of each other. Consider sentence A: ‘either O1 or (not-M and not-O2)’. Conjoined with O2, A entails O1. But O2 alone does not entail O1. So A is directly verifiable. Therefore, since M conjoined with A entails O1, which is not entailed by A alone, M is indirectly verifiable. Various repairs have been attempted; none has succeeded.
See also LOGICAL POSITIVISM , MEANING, VERIFICATIONISM , VIENNA CIRCL. E.L.