Full Idea
A space between three lines is the real as well as nominal essence of a Triangle; it being not only the abstract idea to which the name is annexed, but the very Essentia or Being of the thing itself, that foundation from which all its properties flow.
Gist of Idea
A space between three lines is both the nominal and real essence of a triangle, the source of its properties
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.03.18)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.418
A Reaction
Highly significant, coming from a famous doubter of essences. It seems to me that Locke would accept that we know have the essences of innumberable physical entities, which seemed impossible in his day.
Related Ideas
Idea 13432 The essence of a circle is the equality of its radii [Leibniz]
Idea 13073 To understand the properties we must know the essence, as with a circle [Spinoza]