es which are covered by either of the two bounding
moments. Thus the boundary of a duration consists of two momentary
three-dimensional spaces. An event will be said to 'occupy' the
aggregate of event-particles which lie within it.

Two events which have 'junction' in the sense in which junction was
described in my last lecture, and yet are separated so that neither
event either overlaps or is part of the other event, are said to be
'adjoined.'

This relation of adjunction issues in a peculiar relation between the
boundaries of the two events. The two boundaries must have a common
portion which is in fact a continuous three-dimensional locus of
event-particles in the four-dimensional manifold.

A three-dimensional locus of event-particles which is the common portion
of the boundary of two adjoined events will be called a 'solid.' A solid
may or may not lie completely in one moment. A solid which does not lie
in one moment will be called 'vagrant.' A solid which does lie in one
moment will be called a volume. A volume may be defined as the locus of
the event-particles in which a moment intersects an event, provided that
the two do intersect. The intersection of a moment and an event will
evidently consist of those event-particles which are covered by the
moment and lie in the event. The identity of the two definitions of a
volume is evident when we remember that an intersecting moment divides
the event into two adjoined events.

A solid as thus defined, whether it be vagrant or be a volume, is a mere
aggregate of event-particles illustrating a certain quality of position.
We can also define a solid as an abstractive element. In order to do so
we recur to the theory of primes explained in the preceding lecture. Let
the condition named {sigma} stand for the fact that each of the events
of any abstractive set satisfying it has all the event-particles of some
particular solid lying in it. Then the group of all the {sigma}-primes
is the abstractive element which is associated with the given solid. I
will call this abstractive element the solid as an abstractive element,
and I will call the aggregate of event-particles the solid as a locus.
The instantaneous volumes in instantaneous space which are the ideals of
our sense-perception are volumes as abstractive elements. What we really
perceive with all our efforts after exactness are small events far
enough down some abstractive set belonging to the volume as an
abstractive