 So, as a second step, Whitehead introduced a second relation in the class of spatio-temporal regions next to the relation of extension, namely, the relation of cogredience, based on our intuition of rest or motion. Whitehead’s use of this relation gave rise to a constant k, which allowed him to merge spatial and temporal congruence, and which appeared in his formula for the metric of space-time. When Whitehead equated k with c2 (the square of the speed of light) his metric became equal to the Minkowskian metric.