This book is a study of Leibniz’s Principle of Identity of Indiscernibles, the principle that rules out numerically distinct but perfectly similar things. The Identity of Indiscernibles was a central principle in Leibniz’s philosophy. Leibniz derived it from more basic principles and used it to establish important philosophical theses. Thus this book is about the place and role of the Identity of Indiscernibles in Leibniz’s philosophy. It aims at establishing what Leibniz meant by the Principle of Identity of Indiscernibles, what his arguments for and from it were, and to assess those claims a ... More

This book is a study of Leibniz’s Principle of Identity of Indiscernibles, the principle that rules out numerically distinct but perfectly similar things. The Identity of Indiscernibles was a central principle in Leibniz’s philosophy. Leibniz derived it from more basic principles and used it to establish important philosophical theses. Thus this book is about the place and role of the Identity of Indiscernibles in Leibniz’s philosophy. It aims at establishing what Leibniz meant by the Principle of Identity of Indiscernibles, what his arguments for and from it were, and to assess those claims and arguments. The book argues that Leibniz had a very strong version of the principle, according to which necessarily no possibilia (whether or not they belong to the same possible world) are intrinsically perfectly similar, where this excludes things that differ in magnitude alone. The book discusses Leibniz’s arguments for the Identity of Indiscernibles in the Meditation on the Principle of the Individual, the Discourse on Metaphysics, Notationes Generales, Primary Truths, the letter to Casati of 1689, the correspondence with Clarke, as well as the use of the Identity of Indiscernibles in Leibniz’s arguments against the Cartesian conception of the material world, atoms, absolute space and time, the Lockean conception of the mind as a tabula rasa, and freedom of indifference. The general conclusion of the book is that the Identity of Indiscernibles was a central but inessential principle of Leibniz’s philosophy.