Full Idea
In all cases of sense-data capable of gradual change, we may find one sense-datum indistinguishable from another, and that indistinguishable from a third, while yet the first and third are quite easily distinguishable.
Gist of Idea
When sense-data change, there must be indistinguishable sense-data in the process
Source
Bertrand Russell (Our Knowledge of the External World [1914], 5)
A Reaction
This point is key to the sense-data theory, because it gives them independent existence, standing between reality and subjective experience. It is also the reason why they look increasingly implausible, if they may not be experienced.
Book Reference
Russell,Bertrand: 'Our Knowledge of the External World' [Routledge 1993], p.148