Full Idea
When human beings speak, it is very difficult to suppose that what we hear is not the expression of a thought, as we know it would be if we emitted the same sounds.
Gist of Idea
It is hard not to believe that speaking humans are expressing thoughts, just as we do ourselves
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 2)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.11
A Reaction
This is partly the 'argument from analogy', which seems a bit suspect (induction from a single instance), but it is also the rather undeniable Humean idea that we have a 'natural belief' in other minds, which we could never disbelieve.