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 is to note that they are involved in the modes of realisation of abstractive hierarchies, discussed in the previous chapter. The spatio-temporal relationships, involved in those hierarchies as realised in α, have all a definition in terms of α and of the occasions entrant in α. Thus the entrant occasions lend their aspects to the hierarchies, and thereby convert spatio-temporal modalities into categorical determinations; and the hierarchies lend their forms to the occasions and thereby limit the entrant occasions to being entrant only under those forms. Thus in the same way (as seen in the previous chapter) that every occasion is a synthesis of all eternal objects under the limitation of gradations of actuality, so every occasion is a synthesis of all occasions under the limitation of gradations of types of entry. Each occasion synthesises the totality of content under its own limitations of mode.

In respect to these types of internal relationship between α and other occasions, these other occasions (as constitutive of α) can be classified in many alternative ways. These are all concerned with different definitions of past, present, and future. It has been usual in philosophy to assume that these various definitions must necessarily be equivalent. The present state of opinion in physical science conclusively shows that this assumption is without metaphysical justification, even although any such discrimination may be found to be unnecessary for physical science. This question has already been dealt with in the chapter on Relativity. But the physical theory of relativity touches only the fringe of the various theories which are metaphysically tenable. It is important for my argument to insist upon the unbounded freedom