Full Idea
If all objects had individual essences, there would be no numerical difference without an essential difference. But if there aren't individual essences, there could be two things sharing all essential properties, differing only in accidental properties.
Gist of Idea
There are problems both with individual essences and without them
Source
Penelope Mackie (How Things Might Have Been [2006], 2.5)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.25
A Reaction
Depends how you define individual essence. Why can't two electrons have the same individual essence. To postulate a 'kind essence' which bestows the properties on each electron is to get things the wrong way round.