[New Entry by Gonzalo Rodriguez-Pereyra on June 4, 2025.]
The Identity of Indiscernibles is the thesis that there cannot be numerical difference without extra-numerical difference – that is, there cannot be two objects that differ only numerically, solo numero. It is an important issue in metaphysical discussions of identity, haecceitism/non-haecceitism, and theories of properties (since extra-numerical identity is often explained in terms of properties). Although it is most often referred to as a principle, its axiomatic character is not currently a topic…

Read the full article which is published on Stanford Encyclopedia of Philosophy (external link)