Full Idea
To the essence of anything pertains ...that without which the thing can neither be nor be conceived, and which in its turn cannot be nor be conceived without the thing.
Gist of Idea
Essence gives existence and conception to things, and is inseparable from them
Source
Baruch de Spinoza (The Ethics [1675], II Def 2)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.45
A Reaction
Note that essence concerns not only what things are, but also our ability to conceive them.