Full Idea
A thing is called necessary either in reference to its essence or its cause.
Gist of Idea
Necessity is in reference to essence or to cause
Source
Baruch de Spinoza (The Ethics [1675], I Pr 33)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.31
A Reaction
I like any proposal that necessity should be 'in reference to' something, rather than being free-standing. I like to add necessary 'for' something, which is often conceptual necessity. Roots are necessary for trees.