Full Idea
For Leibniz, it is the very essence of substances to be sources of activity.
Gist of Idea
Substances are essentially active
Source
report of Gottfried Leibniz (works [1690]) by Nicholas Jolley - Leibniz Ch.2
Book Reference
Jolley,Nicholas: 'Leibniz' [Routledge 2005], p.42
A Reaction
This makes the views of Leibniz sympathetic to modern essentialism (of which I am a fan), because it places active power at the centre of what it is to exist, rather than action being imposed on matter which is otherwise passive.