Full Idea
I believe Mont Blanc itself is a component part of what is actually asserted in the proposition 'Mont Blanc is more than 4000 metres high'; we do not assert the thought, which is a private psychological matter, but the object of the thought.
Gist of Idea
We don't assert private thoughts; the objects are part of what we assert
Source
Bertrand Russell (Letters to Frege [1902], 1904.12.12), quoted by Ray Monk - Bertrand Russell: Spirit of Solitude Ch.4
Book Reference
Monk,Ray: 'Bertrand Russell: Spirit of Solitude' [Vintage 1997], p.166
A Reaction
This would appear to be pretty much externalism about concepts, given that Russell would accept that other people know much more about Mont Blanc than he does, and their knowledge is included in what he asserts.