Gottfried Leibniz held that no two distinct objects can have exactly the same properties. But, Max Black asked, “Isn’t it logically possible that the universe should have contained nothing but two exactly similar spheres? We might suppose that each was made of chemically pure iron, had a diameter of one mile, that they had the same temperature, colour, and so on, and that nothing else existed. Then every quality and relational characteristic of the one would also be a property of the other.” (We might object that the two spheres are discernible because they occupy different positions in space, but...