 This reply raises a prima facie problem for Cook Wilson, because he could not have known, in the sense of ‘knowing p only if p’, that ‘non-Euclidean geometries are inconsistent’, since it has been proved that they are not. He thus unwittingly provided an illustration of the need to account, from his own internalist standpoint, for the sort of error (or ‘false judgement’) committed when one claims to know that p, while it is the case that not-p. For someone to know or to be in error while thinking that one knows would just be two undistinguishable states, since which of the two happens to be the case would depend on some external factors: