There are an indefinite number of types of entity disclosed in this diversification. An attempt in this enquiry to trace the subtlety of nature would only blur the main argument. Accordingly we confine attention to five modes of diversification which are chiefly important in scientific theory. These types of entities are: (i) events, (ii) percipient objects, (iii) sense-objects, (iv) perceptual objects, (v) scientific objects. These are five radically distinct types of entities yielded by five distinct procedures; and their only common quality as entities is that they are all alike subjects yielded for our knowledge by our perceptions of nature. 13.3 The entities which are the product of any one mode of diversification of nature will be called elements, or aspects, of nature; each such entity is one natural element. Thus each mode of diversification produces natural elements of a type peculiar to itself. One mode of diversification is not necessarily more abstract than another. Objects can be looked on as qualities of events, and events as relations between objects, or—more usefully—we can drop the metaphysical and difficult notion of inherent qualities and consider the elements of different types as bearing to each other relations. There are accordingly two main genera of relations to be distinguished, namely 'homogeneous' relations which relate among themselves natural elements of the same type, and 'heterogeneous' relations which relate natural elements of different types. 13.4 Another way of considering the diversification of nature is to emphasise primarily the relations between natural elements. Thus those elements are what is perceived in nature as thus related. In other words the relations are treated as fundamental and the natural elements are introduced as in their capacity of relata. But of course this is merely another mode of expression, since relations and relata imply each other. 14. Events. 14.1 Events are the relata of the fundamental homogeneous relation of 'extension.' Every event extends over other events which are parts of itself, and every event is extended over by other events of which it is part. The externality of nature is the outcome of this relation of extension. Two events are mutually external, or are 'separate,' if there is no event which is part of both. Time and space both spring from the relation of extension. Their derivation will be considered in detail in subsequent parts of this enquiry. It follows that time and space express relations between events. Other natural elements which are not events are only in time and space derivatively, namely, by reason of their relations to events. Great confusion has been caused to the philosophy of science by this neglect of the derivative nature of the spatial and temporal relations of objects of various types.