Full Idea
It is hard to individuate sense-data, saying where one ends and the next begins, and hard to say whether they can change; are they substances, qualities, events, or what?
Gist of Idea
Where do sense-data begin or end? Can they change? What sort of thing are they?
Source
A.R. Lacey (A Dictionary of Philosophy [1976], p.196)
Book Reference
Lacey,A.R.: 'A Dictionary of Philosophy' [RKP 1976], p.196
A Reaction
The problem is not that these questions are unanswerable. The answer seems to be either that they are physical and external, or that they are mental and internal, and that there is no ontological space for them between the two.