Full Idea
There are no two individuals indiscernible from one another - leaves, or drops of water, for example. This is an argument against atoms, which, like the void, are opposed to the principles of a true metaphysic.
Gist of Idea
Atomism is irrational because it suggests that two atoms can be indistinguishable
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], 4.04)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.216