Rectilinear routes are momental routes and stations are vagrant routes.

Rectilinear routes are routes which in a sense lie in rects.

Any two event-particles on a rect define the set of event-particles which lie between them on that rect.

Let the satisfaction of the condition {sigma} by an abstractive set mean that the two given event-particles and the event-particles lying between them on the rect all lie in every event belonging to the abstractive set.

The group of {sigma}-primes, where {sigma} has this meaning, form an abstractive element.