Full Idea
In Leibniz's view, the essence of a thing is fundamentally the real possibilities of that thing.
Gist of Idea
The essence of a thing is its real possibilities
Source
report of Gottfried Leibniz (works [1690]) by Cover,J/O'Leary-Hawthorne,J - Substance and Individuation in Leibniz 4.3.3
Book Reference
Cover,J/O'Leary-Hawthorne,J: 'Substance and Individuation in Leibniz' [CUP 1999], p.169
A Reaction
Note that the essences are individual. On the whole I would prefer Leibniz in his own words, but this is too good to lose (..but see Idea 12981). It is the aspect of Leibniz that fits perfectly with modern scientific essentialism.
Related Idea
Idea 12981 Essence is just the possibility of a thing [Leibniz]