Full Idea
It is clear that modality is a prima-facie threat to the usual kind of naturalistic-causal-empiricist theory of knowledge.
Gist of Idea
Necessity and possibility are big threats to the empiricist view of knowledge
Source
Colin McGinn (Logical Properties [2000], Ch.4)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.84
A Reaction
This is why modern empiricists spend of a lot of energy on trying to analyse counterfactuals and laws of nature. Rationalists are much happier to assert necessities a priori, but then they often don't have much basis for their claims.