These abstractions exhibit features present in that fact, and are thus not mere fairy tales; but they must not be construed as presenting separate and separable characters whose accidental union makes nature. Nature is not built up of logical elements, but logical elements are obtained by discrimination and abstraction from nature. This I take to be Professor Whitehead's fundamental contention, though perhaps he would object to the way in which I have stated it. If I am right in my understanding of him, again it seems that he is attacking his problem in the only hopeful way. There may be criticism in detail of his results, but such criticism should not imply any dissatisfaction with his starting point or his general method.
The general result that he obtains presents us with a fascinating world-view. Nature is marching on; that is, events are continuously happening. An event never happens again. There is never any reversal in this process. But an event is characterised, and the characters involved are recognised in other events. These recognisable characters are objects and relations. The irreversibility of the stream of events is the passage of nature, which we may here call the sequence of nature. In nature there are no alternative sequences. But sequence is not all that we mean by 'time'. In 'time' events not only follow other events and precede still others; but they also are simultaneous with yet others. Now while there are not alternative sequences in nature, they are alternative simultaneities. Hence, if we mean by time a system of relations which includes both sequence and simultaneities, then there is not just one time in nature. There are many times, or as Professor Whitehead calls them, many 'time-systems' or 'families of durations', a 'duration' being a slab of nature characterized by simultaneity and also exhibiting passage. Or, to use Professor Whitehead's own language, the unity of a duration "is expressed by the concept of simultaneity" (p. 53). I take that it that for him 'simultaneity' is something ultimate and unanalysable. Einstein defines simultaneity by assuming the constancy of the velocity of light in vacuo, and by using a system of light signals to synchronise clocks; and then events which happen in any two places in any system at rest are defined as simultaneous if the clock readings at these two places are identical. But Whitehead finds simultaneity as a property of certain wholes of nature, such wholes being durations.
Whitehead's definition of a family of durations need not be quoted here. The point to be noted is that durations of different families intersect; and if in each family a class or series of durations be se-
