Full Idea
If we are to avoid a perfectly gratuitous assumption, we must dispense with the subject as one of the actual ingredients of the world; but when we do this, the possibility of distinguishing the sensation from the sense-datum vanishes.
Clarification
The 'subject' is the self, or the ego, or stable personal identity
Gist of Idea
We cannot assume that the subject actually exists, so we cannot distinguish sensations from sense-data
Source
Bertrand Russell (The Analysis of Mind [1921], Lec. VIII)
Book Reference
Russell,Bertrand: 'The Analysis of Mind' [Routledge 1995], p.142
A Reaction
This is the reason why Russell himself rejected sense-data. It is more normal, I think, to reject them simply as being superfluous. If the subject can simply perceive the sense-data, why can't they just perceive the object more directly?