 Leibniz is committed to the principle of the identity of indiscernibles. This principle states, roughly, that it is impossible for there to be two numerically distinct substances that are qualitatively identical. In discussing this principle Leibniz sometimes supports it by appealing to an anecdote and other empirical considerations. In the anecdote he recalls discussing the principle with an “ingenious gentleman” who thought he could find two perfectly identical leaves. Princess Sophia, who was observing the conversation, “defied him to do it; he ran all over the garden a long time to look for some but it was to no purpose” (4th letter to Clarke, G 7:372; AG 327–8). Leibniz also defends the principle by appealing to the empirical results of the microscopists: objects that can seem qualitatively identical to the naked eye (two drops of water, or two drops of milk) are shown to be different when viewed under the microscope. Neither the anecdote nor the appeal to microscopes is intended to be anything like a rigorous proof of the principle of the identity of indiscernibles. A rigorous proof would show how the principle is derived a priori from other principles such as the principle of sufficient reason.