Let us start by considering for a moment our ordinary idea of "Time." To start with we associate it with clocks and next, if we go a step further back, with the movement of the earth relative to the sun and stars. A clock is merely a mechanical device for subdividing into equal parts of suitable size the intervals between successive recurrences of certain astronomical events. In fact our ordinary ideas of time are determined by a wholly fortuitous arrangement of the component parts of the Solar System. If the masses etc. were other than they are, our day and year would be altered accordingly. It is quite conceivable that in some highly complex system of several "suns" moving under the influence of their mutual attractions and attended each by its own sub-system of satellites, there might be a world from which all the observable astronomical phenomena would be so complicated that its inhabitants could detect no regularity in them at all. If, for instance, any given astronomical grouping of the observable bodies only recurred once in a hundred generations of the inhabitants, the measurement of time from astronomical data would be scarcely practicable. A similar state of things would result if the average life of a man on earth lasted about ten minutes. Again we know that the regularity of the changes in our system is really only apparent, for all the motions by which we habitually measure time are gradually altering under the influence of tidal friction. So we see that all our ordinary ideas of time are based on the fissiparous assumption that certain distributions of matter will occur regularly; that is to say in such a manner that if we could observe any two successive cycles simultaneously they would appear coincident. The same can be shown to apply to any other system of time measurement which we can substitute for the observation of astronomical phenomena. This is so because, apart from all other reasons, every conceivable method must be based on the assumption that the properties of matter are invariable. But these seem to be functions of the properties of ether and since the solar system is certainly, and the whole universe probably, moving through ether-filled space, this means that our methods of time measurement must ultimately be based on the assumption that the ether is homogeneous. Very probably it is; but there is no reason why it should be--on _a priori_ grounds. Now M. Bergson has been at pains to discriminate between this time "of succession" which we know and true time--the time "of duration." His view, as I understand it, is that the succession of events or "spatial simultaneities" by which we _measure_ time no more _is_ time than the succession of marks on a foot-rule _is_ the material which we measure with it. What we actually experience as time does not necessarily correspond with the spatial recurrences which measure it.