Full Idea
It is very difficult to deal with the theory of error without assuming the existence of the non-existent.
Gist of Idea
The theory of error seems to need the existence of the non-existent
Source
Bertrand Russell (The Philosophy of Logical Atomism [1918], §IV.3)
Book Reference
Russell,Bertrand: 'Russell's Logical Atomism', ed/tr. Pears,David [Fontana 1972], p.81
A Reaction
This problem really bothered Russell (and Plato). I suspect that it was a self-inflicted problem because at this point Russell had ceased to believe in propositions. If we accept propositions as intentional objects, they can be as silly as you like.