Full Idea
For there to be a certain persisting law which involves the future states of that which we conceive as one and the same continuant, this is what I say constitute's a substance's identity.
Gist of Idea
Identity of a substance is the law of its persistence
Source
Gottfried Leibniz (works [1690], G II:264), quoted by David Wiggins - Sameness and Substance 3.1
Book Reference
Wiggins,David: 'Sameness and Substance' [Blackwell 1980], p.85
A Reaction
This is a key remark for those who thing 'persistence conditions' are basic to metaphysics. I'm not so sure.