Full Idea
It is metaphysically necessary that Hesperus is Phosphorus, but not logically necessary, since logical deduction could not reveal its truth, and it is not epistemologically necessary, as the ancient Greeks didn't know the identity. (Natural necessity?)
Gist of Idea
Is 'Hesperus = Phosphorus' metaphysically necessary, but not logically or epistemologically necessary?
Source
Gabriel M.A. Segal (A Slim Book about Narrow Content [2000], 1.6)
Book Reference
Segal,Gabriel M.A.: 'A Slim Book about Narrow Content' [MIT 2000], p.14