Full Idea
If a rational man believes he has at least some other false beliefs, it follows that a rational man knows that at least one of his beliefs is false (the one believed false, or this new belief).
Gist of Idea
If you believe that some of your beliefs are false, then at least one of your beliefs IS false
Source
Gilbert Harman (Thought [1973], 7.2)
Book Reference
Harman,Gilbert: 'Thought' [Princeton 1977], p.119