Full Idea
If two individuals were perfectly similar and equal and, in short, indistinguishable in themselves, there would be no principle of individuation.
Gist of Idea
If two individuals could be indistinguishable, there could be no principle of individuation
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.27)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.230
A Reaction
This seems to be the main motive for Leibniz's unusual claim that there cannot be two indiscernible individuals, but it looks suspiciously like an a priori claim made about what should be an a posteriori discovery. Are electrons distinguishable?