Full Idea
Substantial form, or soul, is the principle of unity and duration, matter is that of multiplicity and change
Gist of Idea
Form or soul gives unity and duration; matter gives multiplicity and change
Source
Gottfried Leibniz (Conspectus libelli (book outline) [1678], A6.4.1398-9), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 2
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.55
A Reaction
Leibniz was a fan of the unfashionable Aristotle, and tried to put a spin on his views consonant with contemporary Hobbesian mechanistic views. Oddly, he likes the idea that 'form' is indestructable, which I don't understand.