Full Idea
Essentialists do not accept the standard position, which says necessity is a priori, and contingency is a posteriori. They have a radically new category: the necessary a posteriori. The laws of nature are, for example, both necessary and a posteriori.
Clarification
'Necessary a posteriori' has to be true, but is known from experience
Gist of Idea
Essentialists say natural laws are in a new category: necessary a posteriori
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.6)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.109
A Reaction
Based on Kripke. I'm cautious about this. Presumably God, who would know the essences, could therefore infer the laws a priori. The laws may follow of necessity from the essences, but the essences can't be known a posteriori to be necessary.