Full Idea
The essence of the soul is to represent bodies. ...The soul and the idea of the body do not signify the same thing. For the soul remains one and the same, while the idea of the body perpetually changes as the body itself changes.
Gist of Idea
Soul represents body, but soul remains unchanged, while body continuously changes
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1699.03.24/04.03)
Book Reference
Leibniz,Gottfried: 'Leibniz Selections', ed/tr. Wiener,Philip P. [Scribners 1951], p.161
A Reaction
This seems to rest on the Cartesian Ego, as the essence of mind which does not change. And yet elsewhere he describes the Ego as a mere abstraction from introspected mental life.