Full Idea
Why should we accept that necessities can only be known a priori? Prima facie, some necessities are known empirically; for example, that water is necessarily H2O, and that Hesperus is necessarily Phosphorus.
Clarification
'Prima facie' means at first glance. See Idea 4972 for Hesperus/Phosphorus.
Gist of Idea
Why should necessities only be knowable a priori? That Hesperus is Phosporus is known empirically
Source
Michael Devitt (There is no a Priori [2005], §2)
Book Reference
'Contemporary Debates in Epistemology', ed/tr. Steup,M/Sosa,E [Blackwell 2005], p.107
A Reaction
An important question, whatever your view. If the only thing we can know a priori is necessities, it doesn't follow that necessities can only be known a priori. It gets interesting if we say that some necessities can never be known a priori.