Full Idea
The soul, properly and accurately speaking, is not a substance, but a substantial form, or the primitive form existing in substances, the first act, the first active faculty.
Gist of Idea
The soul is not a substance but a substantial form, the first active faculty
Source
Gottfried Leibniz (Letters to Fardella [1690], A6.4.1670), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 2
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.92
A Reaction
In all of Leibniz's many gropings towards what is at the heart of a unified object, I pounce on the phrase "the first active faculty" as the one that suits me. I take that to be a 'power'. It has two characteristics - it is active, and it is basic.