Full Idea
There is no reason why I should not know of the existence of something with which nobody is acquainted.
Gist of Idea
I can know the existence of something with which nobody is acquainted
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 4)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.23
A Reaction
This sort of realist claim (which he goes on to say results from inferences from descriptions) is needed to save empiricism from the absurdities of Berkeley and (dare I say it?) Quine. The Kantian Ego is a candidate.