Full Idea
By the force I give to substances, I understand a state from which another state follows, if nothing prevents it. ...I dare say that without force, there would be no substance.
Gist of Idea
Force in substance makes state follow state, and ensures the very existence of substance
Source
Gottfried Leibniz (Letters to Lelong [1712], 1712), quoted by Cover,J/O'Leary-Hawthorne,J - Substance and Individuation in Leibniz 7.1
Book Reference
Cover,J/O'Leary-Hawthorne,J: 'Substance and Individuation in Leibniz' [CUP 1999], p.254
A Reaction
[the whole quote is interesting] This remark, more than any other I have found, places force at the centre of Leibniz's metaphysics. He is using it to resist Spinoza's one-substance view.