In the first place, mathematics furnishes us with _one_ type of this kind of prefiguring. The very movement by which we draw the circumference of a circle on a sheet of paper generates all the mathematical properties of this figure: in this sense an unlimited number of theorems can be said to pre-exist within the definition, although they will be spread out in duration for the mathematician who deduces them. It is true that we are here in the realm of pure quantity and that, as geometrical properties can be expressed in the form of equations, it is easy to understand how the original equation, expressing the fundamental property of the figure, is transformed into an unlimited number of new ones, all virtually contained in the first. On the contrary, physical phenomena, which succeed one another and are perceived by our senses, are distinguished by quality not less than by quantity, so that there would be some difficulty in at once declaring them equivalent to one another. But, just because they are perceived through our sense-organs, we seem justified in ascribing their qualitative differences to the impression which they make on us and in assuming, behind the heterogeneity of our sensations, a homogeneous physical universe. Thus, we shall strip matter of the concrete qualities with which our senses clothe it, colour, heat, resistance, even weight, and we shall finally find ourselves confronted with homogeneous extensity, space without body. The only step then remaining will be to describe figures in space, to make them move according to mathematically formulated laws, and to explain the apparent qualities of matter by the shape, position, and motion of these geometrical figures. Now, position is given by a system of fixed magnitudes and motion is expressed by a law, i.e. by a constant relation between variable magnitudes; but shape is a mental image, and, however tenuous, however transparent we assume it to be, it still constitutes, in so far as our imagination has, so to speak, the visual perception of it, a concrete and therefore irreducible quality of matter. It will therefore be necessary to make a clean sweep of this image itself and replace it by the abstract formula of the movement which gives rise to the figure. Picture then algebraical relations getting entangled in one another, becoming objective by this very entanglement, and producing, by the mere effect of their complexity, concrete, visible, and tangible reality,--you will be merely drawing the consequences of the principle of causality, understood in the sense of an actual prefiguring of the future in the present. The scientists of our time do not seem, indeed, to have carried abstraction so far, except perhaps Lord Kelvin. This acute and profound physicist assumed that space is filled with a homogeneous and incompressible fluid in which vortices move, thus producing the properties of matter: these vortices are the