26. Duality of Nature. 26.1 There are two sides to nature, as it were, antagonistic the one to the other, and yet each essential. The one side is development in creative advance, the essential becomingness of nature. The other side is the permanence of things, the fact that nature can be recognised. Thus nature is always a newness relating objects which are neither new nor old. 26.2 Perception fades unless it is equally stimulated from both sides of nature. It is essentially apprehension of the becomingness of nature. It requires transition, contrast, and newness, and immediacy of happening. Thus essentially perception is an awareness of events in the act of passing into what has never yet been. But equally perception requires recognition. Now electrons—in so far as they are ultimate scientific objects and if they are such objects—do not satisfy the complete condition for recognisability. 26.3 Such ultimate scientific objects embody what is ultimately permanent in nature. Thus they are the objects whose relations in events are the unanalysable expression of the order of nature. But the recognition in perception requires the recurrence of the ways in which events pass. This involves the rhythmic repetition of the characters of events. This permanence of rhythmic repetition is the essential character of molecules, which are complex scientific objects. There is no such thing as a molecule at an instant. A molecule requires a minimum of duration in which to display its character. Similarly physical objects are steady complexes of molecules with an average permanence of character throughout certain minimum durations. 26.4 Thus the recognition which is involved in perception is the reason for the importance in physical science of Lorentz's hierarchy of microscopic and macroscopic equations. 26.5 The further consideration of objects, in particular their instantaneous spatial positions and the quantitative distribution of material through space, is resumed in Part IV, after the theory of space and time has been established. [5]Cf. Chapters XIV and XV of Part IV. PART III THE METHOD OF EXTENSIVE ABSTRACTION CHAPTER VIII PRINCIPLES OF THE METHOD OF EXTENSIVE ABSTRACTION 27. The Relation of Extension,Fundamental Properties. 27.1 The fact that event extends over event will be expressed by the abbreviation . Thus '' is to be read 'extends over' and is the symbol for the fundamental relation of extension. 27.2 Some properties of essential for the method of extensive abstraction are, (i) implies that is distinct from , namely, 'part' here means 'proper part': (ii) Every event extends over other events and is itself part of other events: the set of events which an event e extends over is called the set of parts of : (iii) If the parts of are also parts of and and are distinct, then : (iv) The relation is transitive, i.e. if and , then : (v) If , there are events such as where and :