  Thus formulated, the actual truth of the Principle seems unproblematic for medium-sized objects, such as rocks and trees, for they are complex enough to have distinguishing or individuating features, and hence may always be distinguished by some slight physical difference. But fundamental principles are widely held to be non-contingent. We might require, therefore, that the Principle should hold even for hypothetical cases of qualitatively identical medium sized objects (e.g., clones which, contrary to fact, really are molecule for molecule replicas). In that case, we shall need to distinguish such objects by their spatial relations to other objects (e.g., where they are on the surface of the planet). In that case the Principle is consistent with a universe in which there are three qualitatively identical spheres A, B, and C where B and C are 3 units apart, C and A are 4 units apart and A and B are 5 units apart. In such a universe, A’s being 5 units from B distinguishes it from C, and A’s being 4 units from C distinguishes it from B. The Principle often gets called into question, however, when we consider qualitatively identical objects in a symmetrical universe. Consider, for instance, a perfectly symmetrical universe consisting solely of three qualitatively identical spheres, A, B and C, each of which is the same distance, 2 units, away from the others. In this case there seems to be no property which distinguishes any of the spheres from any of the others. Some would defend the Principle even in this case by claiming that there are properties such as being that very object A. Call such a property a thisness or haecceity.