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)
Book Reference
Russell,Bertrand: 'Our Knowledge of the External World' [Routledge 1993], p.148
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.