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Full Idea
There is some difficulty in deciding what is to be considered one sense-datum: often attention causes divisions to appear where, so far as can be discovered, there were no divisions before.
Gist of Idea
Individuating sense-data is difficult, because they divide when closely attended to
Source
Bertrand Russell (The Relation of Sense-Data to Physics [1914], §II)
Book Ref
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.142
A Reaction
This was, I suspect, why Russell had dropped the idea of sense-data by 1921. He does, however, say that they are the last unit in analysis, rather than being the most basic unit of perception. In other words, they are purely theoretical.