Full Idea
Positivists prefer the sense-datum vocabulary because it is more inclusive than physical object vocabulary; it can report after-images, hallucinations, illusions and bodily sensations, as well as veridical perceptions.
Clarification
A 'veridical' thing contains truth
Gist of Idea
Positivists prefer sense-data to objects, because the vocabulary covers both illusions and perceptions
Source
report of A.J. Ayer (Language,Truth and Logic [1936]) by Howard Robinson - Perception IX.4
Book Reference
Robinson,Howard: 'Perception' [Routledge 2001], p.227
A Reaction
The assumption of this is that illusions and perceptions are frequently indistinguishable, but that is just nonsense. Illusions usually appeal to one sense only, when you are ill, and in an unclear way. Sensible people know objects when they see them.