 In fact, there are now two principles: the old one, which was made immune to revision by considering it as a conventional principle and which serves to construct geometrical space via the transformation group that is the mathematical expression of the psychological principle of the free mobility of “rigid” bodies, and the new one, where the invariance concerns differential equations and where the geometry could now be rooted in the Lorentz group at the price of accepting a four-dimensional spacetime with indefinite metric (see Gray 2012: 111; Walter 2009). So, it seems that Poincaré’s old conventional construction of space where he prefers Euclidean geometry on the basis of considerations of simplicity and commodity must be subsumed to a new convention on space-time. Indeed, Poincaré never abandons Galilean spacetime but it is not even clear if before 1912 the principle of special relativity with Lorentz covariance (instead of with Galilean covariance) was confirmed well enough to be unassailable by experiment and to make the principle with Galilean covariance unfruitful (Walter 2009). This situation is what Larry Sklar has called “transient” underdetermination,