The genetic or empirical school tries to reduce the relations of extension to more or less complex relations of succession in duration. The relations of situation in space are defined as reversible relations of succession in duration. But succession in duration is not reversible. Pure duration is the form of succession of conscious states when one refrains from reflectively setting up a distinctness between the present state and former states. This does not mean being wholly absorbed in the passing sensation or idea, nor forgetting former states; but it means organizing them instead of juxtapositing them; they become like the notes of a melody, which, though they succeed each other, are apperceived in each other; they interpenetrate like the parts of a living being. Succession, then, can be conceived without distinctness, as a mutual penetration, a solidarity, an intimate organization of elements each of which, representative of the whole, is distinguished and isolated therefrom only for a thought capable of abstraction. We introduce the idea of space into our representation of pure succession; we so juxtaposit our states of consciousness as to perceive them simultaneously, not in, but beside each other; we project time upon space, we express duration in terms of extension. Succession then takes the form of a continuous line or of a chain, whose parts touch without interpenetration, which implies a simultaneous before and after instead of a successive--that is, a simultaneous succession, which is a contradiction. Now, when the genetic school defines the relations of situation in space as reversible relations of succession in duration, it represents succession in duration in this self-contradictory way. You cannot make out an order among terms without distinguishing the terms and comparing the _places_ they occupy, without perceiving them, therefore, as juxtaposited. Then to make out an order in the terms of a succession is to make the succession a simultaneity. So this attempt to represent space by means of time presupposes the representation of space. Of space in three dimensions, moreover; for the representation of two dimensions--that is, of a line--implies that of three dimensions: to perceive a line is to place oneself outside it and account for the void surrounding it. Pure duration is nothing but a succession of qualitative changes fusing, interpenetrating, without outlines or tendency to externality by interrelation, without any kinship with number. Pure duration is pure heterogeneity.