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