Note 125 — Introductory material
Reading edition
The homogeneity of space and the relativity of position, therefore, are presupposed in the qualitative spatial comparison with which projective Geometry deals. The latter, as we saw, is also the basis of the principle of duality. But these properties, as I shall now endeavour to prove, belong of necessity to any form of externality, and are thus à priori properties of all possible spaces. To prove this, however, we must first define the notion of a form of externality in general.