Della Rocca invites us to consider the hypothesis that where we ordinarily think there is a single sphere in fact there are many identical collocated spheres, made up of precisely the same parts. (If they were not made up of the same parts then the mass of the twenty spheres would be twenty times that of one sphere, resulting in an empirical difference between the twenty sphere hypothesis and the one sphere hypothesis.)  Intuitively this is absurd, and it is contrary to the Principle, but he challenges those who reject the Principle to explain why they reject the hypothesis. If they cannot, then this provides a case for the Principle. He considers the response that the Principle should be accepted only in the following qualified form: