Full Idea
Many assert that that without which a thing cannot be nor be conceived, belongs to the essence of that thing.
Gist of Idea
The essence of a thing is what is required for it to exist or be conceived
Source
Baruch de Spinoza (The Ethics [1675], II Pr 10)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.89
A Reaction
This is one Aristotelian idea that won't go away, despite the seventeenth century onslaught. It seems obvious that natural kinds, natural objects and human artefacts have properties that can be divided into essential and non-essential.