Full Idea
The essence of a thing was said to be that without which the thing could neither be, nor be conceived to be.
Gist of Idea
The essence is that without which a thing can neither be, nor be conceived to be
Source
John Stuart Mill (System of Logic [1843], 1.6.2)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.123
A Reaction
Fine cites this as the 'modal' account of essence, as opposed to the 'definitional' account.