Now, if the development of the notion of causality, understood in the sense of necessary connexion, leads to the Spinozistic or Cartesian conception of nature, inversely, all relation of necessary determination established between successive phenomena may be supposed to arise from our perceiving, in a confused form, some mathematical mechanism behind their heterogeneity. We do not claim that common sense has any intuition of the kinetic theories of matter, still less perhaps of a Spinozistic mechanism; but it will be seen that the more the effect seems necessarily bound up with the cause, the more we tend to put it in the cause itself, as a mathematical consequence in its principle, and thus to cancel the effect of duration. That under the influence of the same external conditions I do not behave to-day as I behaved yesterday is not at all surprising, because I _change,_ because I _endure._ But things considered apart from our perception do not seem to endure; and the more thoroughly we examine this idea, the more absurd it seems to us to suppose that the same cause should not produce to-day the effect which it produced yesterday. We certainly feel, it is true, that although things do not endure as we do ourselves, nevertheless there must be some reason why phenomena are seen to _succeed_ one another instead of being set out all at once. And this is why the notion of causality, although it gets indefinitely near that of identity, will never seem to us to coincide with it, unless we conceive clearly the idea of a mathematical mechanism or unless some subtle metaphysics removes our very legitimate scruples on the point. It is no less obvious that our belief in the necessary determination of phenomena by one another becomes stronger in proportion as we are more inclined to regard duration as a subjective form of our consciousness. In other words, the more we tend to set up the causal relation as a relation of necessary determination, the more we assert thereby that things do not _endure_ like ourselves. This amounts to saying that the more we strengthen the principle of causality, the more we emphasize the difference between a physical series and a psychical one. Whence, finally, it would result (however paradoxical the opinion may seem) that the assumption of a relation of mathematical inherence between external phenomena ought to bring with it, as a natural or at least as a plausible consequence, the belief in human free will. But this last consequence will not concern us for the moment: we are merely trying here to trace out the first meaning of the word causality, and we think we have shown that the prefiguring of the future in the present is easily conceived under a mathematical form, thanks to a certain conception of duration which, without seeming to be so, is fairly familiar to common sense. [Sidenote: Prefiguring, as having an idea of a future act which we cannot realize without effort, does not involve necessary determination.]