The genetic or empirical theory of space perception regards the sensations by which we succeed in forming the notion of space as themselves unextended and purely qualitative; extension results from their synthesis, as water results from the combination of two elements. Bergson remarks that the fact that water is neither oxygen nor hydrogen nor merely both is just the fact that we embrace the multiplicity of atoms in a single apperception. Eliminate the mind which operates this synthesis and you will at the same time annihilate the water qualities so far as they are other than oxygen and hydrogen qualities; you will, that is, annihilate the aspect under which the synthesis of elementary parts is presented to our consciousness. For space to arise from the coexistence of non-spatial qualities, an act of the mind is necessary, embracing them all together and juxtapositing them--an act which is a Kantian _a priori_ form of sensibility. This act is the conception of an empty homogeneous medium. It is a principle of differentiation other than qualitative differentiation, enabling us to distinguish qualitatively identical simultaneous sensations. Without this principle, we should have perception of the extended, but we should not have conception of space. That is, simultaneous sensations are never absolutely identical, because the organic elements stimulated are not identical. There are no two points of a homogeneous surface that produce the same impression on sight and touch. So there is a real qualitative difference between any two simultaneous points. This, Bergson says, is enough to give us perception of the extended. But the conception of space is _en outre_. The higher one rises in the series of intelligent beings, the more clearly the independent idea of a homogeneous space stands out. Space is not so homogeneous for the animal as for us. Directions are not purely geometrical; they have their quality. We ourselves distinguish our right and left by a natural feeling. We cannot define them. Now, the faculty of conceiving a space without quality is not at all an abstraction; on the contrary, to abstract presupposes the intuition of a homogeneous medium. We know two realities of different order, one heterogeneous, that of sensible qualities, the other homogeneous, which is space. The latter enables us to make sharp distinctions, to count, to abstract, perhaps even to speak. Everybody regards time as an indefinite homogeneous medium, and yet everybody regards it as different from space. Is one, then, reducible to the other?