The Philosophical Review/Volume 1/Summary: Gerhardt - Leibnitz über das Principium Indiscernibilium
Gerhardt gives an unpublished Latin letter of Leibnitz, Ad. R. P. Cosani Lectorem Theologiæ in Collegio Clementina urbis Romae, in which he refers to a conversation with the Herzogin Sophie von Hannover, which from the letter must have taken place before his great journey through Germany to Italy (1687-1690). The letter maintains that two things can never be found to differ from each other without differing not only by external but by internal marks; e.g. two eggs however like externally will be found to contain some internal differences; nor could two globes, placed on each other in imaginary space, be thought even by an angelic or divine intelligence to be exactly equal. This Leibnitz maintains is confirmed by experiments made in the Herrenhausen garden by the Princess. Also no two human minds, though not differing from each other's kind, could ever be thought to be perfectly similar to each other; the minds of Judas and Christ, e.g. looked at in themselves, could never be pronounced alike. No two things, then, either in mind or in matter are perfectly alike.