Full Idea
The idea of the triangle must involve the affirmation that its three angles are equal to two right angles. Therefore this affirmation pertains to the essence of the idea of a triangle.
Gist of Idea
The idea of a triangle involves truths about it, so those are part of its essence
Source
Baruch de Spinoza (The Ethics [1675], II Pr 49)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.88
A Reaction
This seems to say that the essence is what is inescapable when you think of something. Does that mean that brandy is part of the essence of Napoleon? (Presumably not) Spinoza is ignoring the direction of explanation here.