Full Idea
As for motion, what is real in it is force or power; that is to say, what there is in the present state which carries with it a change in the future. The rest is only phenomena and relations.
Gist of Idea
All that is real in motion is the force or power which produces change
Source
Gottfried Leibniz (Letter to the Editor about Bayle [1698], §13)
Book Reference
Leibniz,Gottfried: 'Philosophical Texts', ed/tr. Woolhouse R/Francks,R [OUP 1998], p.207
A Reaction
This presumably contradicts Newton's concept of inertia, which allows constant motion without force. I always like a reference to powers. What is 'kinetic energy' in this context?