 A first step to go from space to space-time was the replacement of (our intuition of) spatial volumes with (our intuition of) spatio-temporal regions (or events) as the basis of the construction (so that, for example, a point of space-time could be defined as an equivalence class of converging spatio-temporal regions). However, whereas Whitehead had constructed the Euclidean distance based on our intuition of cases of spatial congruence (for example, of two parallel straight line segments being equally long), he now struggled to construct the Minkowskian metric in terms of a concept of spatio-temporal congruence, based on a kind of merger of our intuition of cases of spatial congruence and our intuition of cases of temporal congruence (for example, of two candles taking equally long to burn out).