40. Volumes. 40.1 A 'volume prime' is a prime with the formative condition of being a simple abstractive class which covers all the event-particles inhering in an assigned event and covered by an assigned moment. If there are no such particles, there will be no corresponding volume prime. This formative condition is evidently regular for primes. A 'volume' is the abstractive element deduced from a volume prime. A volume is thus the section of an event made by a moment. 40.2 Any volume is covered by the assigned moment of which mention occurs in its definition. Thus every volume (as here defined) is co-momental. Also a volume only covers those event-particles of which mention occurs in its definition. This set of event-particles is completely characteristic of the volume, and may be considered as the volume conceived as a locus of event-particles. In exactly the same way a solid, or a route, is completely defined by the event-particles which it covers and vice versa. Thus solids and routes can be conceived as loci of event-particles. The concrete event itself is also defined by (or, analysed by) the event-particles inhering in it, and such a set of event-particles defines only one event. Thus an event can be looked on as a locus of event-particles. An event ′ which is part of an event is defined in this way by a set of event-particles which are some of the set defining . This fact is the reason for the confusion of the logical 'all' and 'some' with the physical 'whole' and 'part' which apply solely to events. An event is also uniquely defined by the set of event-particles which form its boundary. CHAPTER XI POINTS AND STRAIGHT LINES 41. Stations. 41.1 The fact that an event is 'cogredient' with a duration is a fundamental fact not to be explained purely in terms of extension. It has been pointed out in Part II that the exact concept of cogredience is 'Here throughout the duration' or 'There throughout the duration.' Let this fundamental relation of finite events to durations be denoted by ',' and let '' mean ' is a finite event which is cogredient with the duration .' 41.2 A 'stationary prime' within a duration is a prime whose formative condition () is that of being a simple abstractive class, such that each of its members extends over events which (i) are inhered in by some assigned event-particle inherent in and (ii) have the relation to . This formative condition is regular for primes. A 'station' within a duration is the abstractive element deduced from a stationary prime within .