Full Idea
If p is a false affirmative proposition ...then it seems obvious that if we believe not-p we do know something true, so belief in not-p must be something which is not mere disbelief. This proves that there are negative propositions.
Gist of Idea
If p is false, then believing not-p is knowing a truth, so negative propositions must exist
Source
Bertrand Russell (Meinong on Complexes and Assumptions [1904], p.75)
Book Reference
Russell,Bertrand: 'Essays in Analysis', ed/tr. Lackey,Douglas [George Braziller 1973], p.75
A Reaction
This evidently assumes excluded middle, but is none the worse for that. But it sounds suspiciously like believing there is no rhinoceros in the room. Does such a belief require a fact?