Full Idea
Even in cases (such as Goldbach's Conjecture) which, if true, are logically necessary, we may be able to conceive the opposite. We can conceive of there being a number which is not the sum of two primes.
Gist of Idea
If Goldbach's Conjecture is true (and logically necessary), we may be able to conceive its opposite
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 3.II)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.66
A Reaction
[attributed to Kneale] Ah, but can we conceive this (as Descartes would say) 'clearly and distinctly'? I can conceive circular squares, as long as I don't concentrate too hard.