Full Idea
The notion of force is as clear as that of action and passion, because it is that from which action follows when nothing prevents it.
Gist of Idea
Clearly, force is that from which action follows, when unimpeded
Source
Gottfried Leibniz (Letters to Paul Pellison-Fontinier [1691], A1.6.226), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 4
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.154
A Reaction
For Leibniz, force seems to be a metaphysical notion, rather than a feature of the physical world. I take it to be the bottom level of explanation, and it equates with Aristotelian form and essence.