Difference (philosophy) - Kant's Critique

Kant's Critique

In his Critique of Pure Reason, Immanuel Kant argues that it is necessary to distinguish between the thing in itself and its appearance. Even if two objects have completely the same properties, if they are at two different places at the same time, they are numerically different:

Identity and Difference.— Thus, in the case of two drops of water, we may make complete abstraction of all internal difference (quality and quantity), and, the fact that they are intuited at the same time in different places, is sufficient to justify us in holding them to be numerically different. Leibnitz regarded phaenomena as things in themselves, consequently as intelligibilia, that is, objects of pure understanding, and in this case his principle of the indiscernible (principium identatis indiscernibilium) is not to be impugned. But, as phaenomena are objects of sensibility, and, as the understanding, in respect of them, must be employed empirically and not purely or transcendentally, plurality and numerical difference are given by space itself as the condition of external phaenomena. For one part of space, although it may be perfectly similar and equal to another part, is still without it, and for this reason alone is different from the latter . It follows that this must hold good of all things that are in the different parts of space at the same time, however similar and equal one may be to another.