Bergson says, then, that the process of duration which we know directly, if it is to be called a series at all, must be described as a series whose “parts” interpenetrate, and this is the first important respect in which non-logical duration differs from a logical series. In “a series” which is used to describe duration not only are the “parts” not distinct but “their relations” are not external in the sense, previously explained, in which logical relations are external to the terms which they relate. A logical term in a logical series can change its position or enter into a wholly different series and still remain the same term. But the terms in a series which has duration (again this is absurd) are what they are just because of their position in the whole stream of duration to which they belong: to transfer them from one position in the series to another would be to alter their whole flavour which depends upon having had just that particular past and no other. As illustration we might take the last bar of a tune. By itself, or following upon other sounds not belonging to the tune, this last bar would not be itself, its particular quality depends upon coming at the end of that particular tune. In a process of duration, then, such as tune, the “later” bars are not related externally to the “earlier” but depend for their character upon their position in the whole tune. In actual fact, of course, the tune progresses continuously, and not by stages, such as distinct notes or bars, but if, for the sake of description, we speak of it as composed of different bars, we must say that any bar we choose to distinguish is modified by the whole of the tune which has gone before it: change its position in the whole stream of sound to which it belongs and you change its character absolutely.