 Perhaps the signal example of this shift is the Law Professor F. K. Schweikart, who in 1818 sent Gauss via Gerling, a colleague and former student of his at the University of Marburg, an account of a geometry quite different from Euclid’s. Schweikart’s geometry was accepted by Gauss, who replied that all the properties of the new geometry could be derived once a value was given for a constant that appeared in Schweikart’s account. But on what grounds Gauss had accepted it, is less clear. As the years went by he came to be entirely confident that there was a new, two-dimensional geometry different from Euclidean plane geometry. This geometry could be described by formulae that he, following Lambert, would have seen were akin to those of spherical geometry. But he did not describe a three-dimensional geometry of this kind, leaving open the possibility that the two-dimensional geometry was some kind of formal, meaningless oddity. On the other hand, in correspondence with Bessel he made it clear that he could not ascribe to Euclidean geometry the certainty he gave to arithmetic, which was a priori. Both he and Bessel kept open the possibility that astronomical regions of space might fail to be Euclidean.