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Full Idea
Motion in itself separated from force is merely relative, and one cannot establish its subject. But force is something real and absolute.
Gist of Idea
Motion alone is relative, but force is real, and establishes its subject
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1688.01.4/14)
Book Ref
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.167
A Reaction
The striking phrase here is that force enables us to 'establish its subject'. That is, force is at the heart of reality, and hence, through causal relations, individuates objects. That's how I read it.