Perceptual space is not a special space in addition to physical space, but physical space which we endow with a special subjective metric.... apart from the definition of congruence in physics and that based on perception, there is no third one derived from pure visualization. Any such third definition is nothing but the definition of physical congruence to which our normative function has adjusted the subjective experience of congruence.

The Philosophy of Space and Time (1928, tr. 1957)