Full Idea
That there should be a persistent law of the series, which involves the future states of that which we conceive to be the same, is exactly what I say constitutes it as the same substance.
Gist of Idea
The law of the series, which determines future states of a substance, is what individuates it
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1704), quoted by Richard T.W. Arthur - Leibniz 4 'Applying'
Book Reference
Arthur, Richard T.W.: 'Leibniz' [Polity 2014], p.96
A Reaction
The 'law of the series' is a bit dubious, but it is reasonable to say that a substance is individuated by its coherent progress of change over time. Disjointed change would imply an absence of substance. The law of the series is called 'primitive force'.