Full Idea
From the point of view of someone with a theory every other theory is false, because it cannot be added to the true theory.
Gist of Idea
If one theory is held to be true, all the other theories appear false, because they can't be added to the true one
Source
Jonathan Dancy (Intro to Contemporary Epistemology [1985], 8.2)
Book Reference
Dancy,Jonathan: 'Introduction to Contemporary Epistemology' [Blackwell 1985], p.115