Full Idea
Logically a sense-datum is an object, a particular of which the subject is aware; it does not contain the subject as a part, as for example beliefs and volitions do.
Clarification
'Volitions' are acts of will
Gist of Idea
Sense-data are objects, and do not contain the subject as part, the way beliefs do
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 very firmly rejects any notion that a sense-datum is mental. It is a left as a strange sort of object which gets as close as it is possible to get to the 'borders' of the mind, without actually becoming part of it.