Full Idea
If privacy is the main objection to introspective data, we shall have to include among such data all sensations; a toothache, for example, is essentially private; a dentist may see the bad condition of your tooth, but does not feel your ache.
Gist of Idea
If we object to all data which is 'introspective' we will cease to believe in toothaches
Source
Bertrand Russell (On Propositions: What they are, and Meaning [1919], §II)
Book Reference
Russell,Bertrand: 'Logic and Knowledge', ed/tr. Marsh,Robert Charles [Routledge 1956], p.294
A Reaction
Russell was perhaps the first to see why eliminative behaviourism is a non-starter as a theory of mind. Mental states are clearly a cause of behaviour, so they can't be the same thing. We might 'eliminate' mental states by reducing them, though.