If an abstractive class be an absolute antiprime, it is evident that the formative condition of 'covering it' is regular for antiprimes. Thus the set of events which are members of the absolute antiprimes which cover some one assigned absolute antiprime constitutes an abstractive element. Such an element will be called a 'moment.' Thus a moment is an abstractive element deduced from the condition of covering an absolute antiprime. Only events of a certain type can be members of an absolute antiprime, namely events which in Part II have been called 'durations.' Only durations can extend over durations, and accordingly all the members of a moment are durations. 33.3 We may conceive of a duration as a sort of temporal thickness (or, slab) of nature[6]. In an absolute antiprime we have a series of temporal thicknesses successively packed one inside the other and converging towards the ideal of no thickness. An absolute antiprime indicates the ideal of an extensionless moment of time. [6]The slab of nature forming a duration is limited in its temporal dimension and unlimited in its spatial dimensions. Thus it represents a finite time and infinite space. Fig. 7. For example let the horizontal line represent the time; and assume nature to be spatially one-dimensional, so that an unlimited vertical line in the diagram represents space at an instant. Fig. 8. Then the area between the unlimited parallel lines and represents a duration. Also the area between and represents another duration which is extended over by the duration bounded by and . But in fig. 7 we have assumed only one time-system, which is the Newtonian hypothesis. Suppose there are many time-systems and consider two such systems and . These are represented by two lines inclined to each other. A duration of time-system is represented by the area between and , and a duration of time-system is represented by the area between and . Two such durations necessarily intersect and also can neither completely extend over the other. These diagrams are crude illustrations of some properties of durations and are in many respects misleading as the sequel will show. The set of moments which inhere in a duration are completely characteristic of that duration, and vice versa. A moment is to be conceived as an abstract of all nature at an instant. No abstractive element can cover a moment except that moment itself. A moment is a route of approximation to all nature which has lost its (essential) temporal extension; thus it is nature under the aspect of a three-dimensional instantaneous space. This is the ideal to which we endeavour to approximate in our exact observations.