Black’s spheres are third grade discriminable because they stand in the symmetric relation of being at least two miles apart, but this example illustrates the objection that third grade discriminability presupposes non-identity (see French 2006).  For suppose we identify the two spheres, treating space as cylindrical, then the geodesic joining the sphere would still be a geodesic and remain the same length. So we could quite naturally say the sphere was at least two miles from itself, unless we analyse that relation negatively as there being no path joining the spheres of less than two miles. But that negative relation only holds in the Black case because the spheres are not identified.