It is then called a 'momental route.' We will not delay over any general consideration of these momental routes, nor is it important for us to proceed to the still wider investigation of vagrant routes in general.

There are however two simple sets of routes which are of vital importance.

One is a set of momental routes and the other of vagrant routes.

Both sets can be classed together as straight routes.

We proceed to define them without any reference to the definitions of volumes and surfaces.
The two types of straight routes will be called rectilinear routes and stations.