Full Idea
Suppose Nixon actually turned out to be an automaton. That might happen. But that is a question about our knowledge. The question of whether he might not have been a human being, given that he is one, is not a question about knowledge.
Gist of Idea
Given that Nixon is indeed a human being, that he might not have been does not concern knowledge
Source
Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 1)
Book Reference
Kripke,Saul: 'Naming and Necessity' [Blackwell 1980], p.47
A Reaction
Given that you are sitting, might you be standing? Yes. Given that you are human, might you be non-human? No. Maybe!