Full Idea
Undeniably, knowledge comes through seeing, but it is a mistake to regard the mere seeing itself as knowledge; if we are so to regard it, we must distinguish the seeing from what is seen; a patch of colour is one thing, and our seeing it is another.
Gist of Idea
Seeing is not in itself knowledge, but is separate from what is seen, such as a patch of colour
Source
Bertrand Russell (The Analysis of Mind [1921], Lec. VIII)
Book Reference
Russell,Bertrand: 'The Analysis of Mind' [Routledge 1995], p.141
A Reaction
This is Russell's 1921 explanation of why he adopted sense-data (but he rejects them later in this paragraph). This gives a simplistic impression of what he intended, which has three components: the object, the 'sensibile', and the sense-datum.