  In his early work on perception Ayer espoused a strict form of phenomenalism, defending the view that statements about material objects are translatable into statements about actual and possible ‘sense-contents’. These latter statements were the ultimate verifiers, forming the basis upon which our empirical world was constructed. Although he later abandoned the reductionism inherent in the translatability requirement (beginning with “Phenomenalism” in 1947), believing that he had been wrong to think that any statement about a physical object could be entailed by a set of statements about sensory experience (sense-data), Ayer continued to hold that our claims about physical objects were justified by reference to such sensory experience. He consistently opposed the view, espoused by Rudolf Carnap, Otto Neurath, and Karl Popper, that the only justifiers were sentences, whether Neurath’s ‘protocols’ or Popper’s ‘basic statements’. His criticism of such views was that the favoured class of statements could not be picked out in the right way without an appeal to relevant experience. So a criterion for membership of the favored class of statements that required only those statements accepted by the scientists of the time to be members of the class was not going to be successful without knowing which sentences were thus accepted, and this, Ayer claimed, could only be known by experience. The alternative of using yet another sentence, one stating that these (p,q,r, …) were the sentences in the relevant class (those accepted by the scientists), would make the foundations of science entirely arbitrary.