Conversely, there is not one truth of reason which does not retain its coefficient of facticity: the alleged transparency of Euclidean geometry is one day revealed as operative for a certain period in the history of the human mind, and signifies simply that, for a time, men were able to take a homogeneous three dimensional space as the 'ground' of their thoughts, and to assume unquestioningly what generalized science will come to consider as a contingent account of space.