Full Idea
The introduction of the term 'sense-datum' is a means of referring to appearances without prejudging the question of what it is, if anything, that they are appearances of.
Gist of Idea
The concept of sense-data allows us to discuss appearances without worrying about reality
Source
A.J. Ayer (Phenomenalism [1947], §1)
Book Reference
Ayer,A.J.: 'Philosophical Essays' [Macmillan 1965], p.131