56.2 If be a material object of a certain sort and be a volume in which is located and be a portion of , then the material object of the same sort as which is located in is called an 'extensive component' of . 56.3 It is by means of the properties of material objects that the atomic properties of objects are combined in mathematical calculations with the extensive continuity of events. Apart from material objects mathematical physics as at present developed would be impossible. For example where the physicist sees the electron as an atomic whole, the mathematician sees a distribution of electricity continuous in time and in space and capable of division into component objects which are also analogous distributions. 57. Stationary Events. 57.1 In order to understand the theory of the motion of material objects, it is first necessary to define the concept of a 'stationary' event. Consider some given time-system , and let denote a volume lying in a certain moment of this time-system. Let be a duration of bounded by moments and , and inhered in by ; so that , , are three parallel moments of the time-system , and lies between and . The volume is the locus of a set of event-particles and each of these event-particles lies in one and only one station of the duration . Also each station of either does not intersect or intersects it in one event-particle only. The assemblage of event-particles lying on stations of d which intersect [namely, each event-particle lying on one of these stations] is the complete set of event-particles analysing[8] an event. Such an event is called stationary in the time-system and stretches throughout the duration . It can also be called 'stationary in ,' since defines the time-system . Every event-particle within the event lies on a station of ; and a station of either has all its event-particles lying within the event or none of them. The volume is the section of the event by the moment . Furthermore if ′ be any other moment of the time-system it lying between and , it intersects the event in a volume ′ which is a geometrical replica of the volume . The moments and which bound the duration are the terminal moments of any event which is stationary in . The stations of lying in the event intersect and in terminal volumes and which are geometrical replicas of and ′. A volume, such as ', in which a moment of intersects an event stationary in is called a 'normal cross-section' of the event. A moment of another time-system which intersects the stationary event in a volume , but does not intersect either of the terminal volumes, is said to intersect it in an 'oblique cross-section.' All the oblique cross-sections of a stationary event which are made by moments of the same time-system are geometrical replicas of each other.