(vi) If and are any two events, there are events such as where and . It follows from (i) and (iv) that and are inconsistent. Properties (ii) and (v) and (vi) together postulate something like the existence of an ether; but it is not necessary here to pursue the analogy. 28. Intersection,Separation and Dissection. 28.1 Two events 'intersect' when they have parts in common. Intersection, as thus defined, includes the case when one event extends over the other, since is transitive. If every intersector of also intersects , then either or and are identical. Events which do not intersect are said to be 'separated.' A 'separated set' of events is a set of events of which any two are separated from each other. 28.2 A 'dissection' of an event is a separated set such that the set of intersectors of its members is identical with the set of intersectors of the event. Thus a dissection is a non-overlapping exhaustive analysis of an event into a set of parts, and conversely the dissected event is the one and only event of which that set is a dissection. There will always be an indefinite number of dissections of any given event. If , there are dissections of of which is a member. It follows that if is part of , there are always events separated from which are also parts of . 29. The Junction of Events. 29.1 Two events and are 'joined' when there is a third event such that (i) intersects both and , and (ii) there is a dissection of of which each member is a part of , or of , or of both. The concept of the continuity of nature arises entirely from this relation of the junction between two events. Two joined events are continuous one with the other. Intersecting events are necessarily joined; but the notion of junction is wider than that of intersection, for it is possible for two separated events to be joined. Two events which are joined have that relation to each other necessary for the existence of one event which extends over them and over no extraneous events. Two events which are both separated and joined are said to be 'adjoined.' 29.2 An event is said to 'injoin' an event when (i) extends over , and (ii) there is some third event which is separated from and adjoined to . Fig. 4. In this definition a property of the boundary of an event first makes its appearance. The assumption that examples of the relation of injunction hold is a long step towards a theory of such boundaries, as the annexed diagram illustrates. It is important to note that injunction has been defined purely in terms of extension. If and is separated from and adjoins , then adjoins .