Full Idea
The sense-datum is an external object of which in sensation the subject is aware; it is true that the sense-datum is in many cases in the subject's body, but the subject's body is as distinct from the subject as tables and chairs are.
Gist of Idea
Sense-data are usually objects within the body, but are not part of the subject
Source
Bertrand Russell (The Relation of Sense-Data to Physics [1914], §IV)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.146
A Reaction
This is probably Russell's clearest statement of the nature of sense-data, which are objects within the subjects body, but are not part of the mind. So once again we come up against the question of their ontology. Are they made of neurons?