37.2 An abstractive class which is prime in respect to the formative condition of 'covering all the elements and abstractive classes constituting some assigned punct' is called an 'absolute prime.' Evidently the condition satisfied by an absolute prime is regular for primes. The abstractive element deduced from an absolute prime is called an 'event-particle.' An event-particle is the route of approximation to an atomic event, which is an ideal satisfied by no actual event. An abstractive class which is antiprime in respect to the formative condition of 'being a member of some assigned punct' is evidently an absolute prime. In fact this set of antiprimes is identical with the set of absolute primes. An event-particle is an instantaneous point viewed in the guise of an atomic event. The punct which an event-particle covers gives it an absolute position in the instantaneous space of any moment in which it lies. Event-particles on a rect lie in the order derived from the puncts which they cover. 37.3 The complete set of event-particles inhering in an event will be called the set 'analysing' that event. A set of event-particles can only analyse one event, and an event can be analysed by only one set of event-particles. An event-particle 'bounds' an event when every event in which the event-particle inheres intersects both and events separated from . The set of event-particles bounding an event is called the 'boundary' of that event. A boundary can only bound one event and every event has a boundary. Event-particles which neither inhere in an event nor bound it are said to lie 'outside' it. The existence of boundaries enables the contact of events to be defined, namely, events are in 'contact' when their boundaries have one or more event-particles in common. The adjunction of events implies contact but not vice versa; since adjunction requires that a solid of the boundaries should be in common. But we define the notion of solid by means of that of adjunction, and not conversely. 37.4 If and are distinct event-particles, there are events separated from each other in which and respectively inhere. Two events intersect if there are event-particles each inhering in both events; and conversely, there are event-particles inhering in both events if they intersect. 37.5 The fact that the instantaneous geometry within a moment is three-dimensional leads to the conclusion that the geometry for all event-particles will be four-dimensional. It is to be noted however that the straight lines for this four-dimensional geometry have so far only been defined for event-particles which are co-momental, namely the rects. Event-particles which are not co-momental will be called 'sequent.' Straight lines of the four-dimensional space joining sequent event-particles will be defined in Chapter XI.