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.