ocus. A station
has accordingly the usual three characters, namely, its character of
position, its extrinsic character as an abstractive element, and its
intrinsic character.

It follows from the peculiar properties of rest that two stations
belonging to the same duration cannot intersect. Accordingly every
event-particle on a station of a duration has that station as its
station in the duration. Also every duration which is part of a given
duration intersects the stations of the given duration in loci which are
its own stations. By means of these properties we can utilise the
overlappings of the durations of one family--that is, of one
time-system--to prolong stations indefinitely backwards and forwards.
Such a prolonged station will be called a point-track. A point-track is
a locus of event-particles. It is defined by reference to one particular
time-system, {alpha} say. Corresponding to any other time-system these
will be a different group of point-tracks. Every event-particle will lie
on one and only one point-track of the group belonging to any one
time-system. The group of point-tracks of the time-system {alpha} is the
group of points of the timeless space of {alpha}. Each such point
indicates a certain quality of absolute position in reference to the
durations of the family associated with {alpha}, and thence in reference
to the successive instantaneous spaces lying in the successive moments
of {alpha}. Each moment of {alpha} will intersect a point-track in one
and only one event-particle.

This property of the unique intersection of a moment and a point-track
is not confined to the case when the moment and the point-track belong
to the same time-system. Any two event-particles on a point-track are
sequential, so that they cannot lie in the same moment. Accordingly no
moment can intersect a point-track more than once, and every moment
intersects a point-track in one event-particle.

Anyone who at the successive moments of {alpha} should be at the
event-particles where those moments intersect a given point of {alpha}
will be at rest in the timeless space of time-system {alpha}. But in any
other timeless space belonging to another time-system he will be at a
different point at each succeeding moment of that time-system. In other
words he will be moving. He will be moving in a straight line with
uniform velocity. We might take this as the definition of a straight
line. Namely, a straight line in the space of time