of events. By
beginning with a duration and ending with it, I mean (i) that the event
is part of the duration, and (ii) that both the initial and final
boundary moments of the duration cover some event-particles on the
boundary of the event.

Every event which is cogredient with a duration extends throughout that
duration.

It is not true that all the parts of an event cogredient with a duration
are also cogredient with the duration. The relation of cogredience may
fail in either of two ways. One reason for failure may be that the part
does not extend throughout the duration. In this case the part may be
cogredient with another duration which is part of the given duration,
though it is not cogredient with the given duration itself. Such a part
would be cogredient if its existence were sufficiently prolonged in that
time-system. The other reason for failure arises from the
four-dimensional extension of events so that there is no determinate
route of transition of events in linear series. For example, the tunnel
of a tube railway is an event at rest in a certain time-system, that is
to say, it is cogredient with a certain duration. A train travelling in
it is part of that tunnel, but is not itself at rest.

If an event e be cogredient with a duration d, and d' be any
duration which is part of d. Then d' belongs to the same time-system
as d. Also d' intersects e in an event e' which is part of e
and is cogredient with d'.

Let P be any event-particle lying in a given duration d. Consider
the aggregate of events in which P lies and which are also cogredient
with d. Each of these events occupies its own aggregate of
event-particles. These aggregates will have a common portion, namely the
class of event-particle lying in all of them. This class of
event-particles is what I call the 'station' of the event-particle P
in the duration d. This is the station in the character of a locus. A
station can also be defined in the character of an abstractive element.
Let the property {sigma} be the name of the property which an
abstractive set possesses when (i) each of its events is cogredient with
the duration d and (ii) the event-particle P lies in each of its events.
Then the group of {sigma}-primes, where {sigma} has this meaning, is an
abstractive element and is the station of P in d as an abstractive
element. The locus of event-particles covered by the station of P in d
as an abstractive element is the station of P in d as a l