When, in the Discourse on Method, Descartes concluded that “...’I’, that is to say, the mind by which I am what I am, is wholly distinct from the body....” (Ital. mine), the way was opened for that dualism between body and soul, extension and thought, which was to become so typical of later French philosophy. Making so sharp a distinction, however, led to occasionalism, where we are confronted with a double series, ---‘physical-psychical’--- whose terms are not necessarily connected, though they work in perfect harmony, thanks to the providence of a thoughtful Deity,--- or should I say, dues ex machine! So that naturally enough, a reaction came: headed indeed by no less a person than Spinoza, who, when he had recovered from the Cartesian spell, became the thoroughgoing monist who declared that “..in nature only one substance exists, and... it is absolutely infinite,” that consequently, “substance thinking and substance extended are one and the same substance, which is now comprehended under this attribute and now under that.” (Ethic, L.Schol. prop. X, II.Schol. prop. VII.). But in the writings of Bergson it is said that we can find both Dualism and Monism expressed; that in fact, he vacillates between one and the other. In the preface to Matter and Memory (Eng. trans.) he says: “This book affirms the reality of spirit and the reality of matter, and tries to determine the relation of the one to the other by the study of a definite example, that of memory. It is, then, frankly dualistic.” He goes on to say, however, that he hopes thereby “..to lessen greatly, if not to overcome, the theoretical difficulties which have always beset dualism, and which cause it ... to be held in small honour among philosophers.” His dualism, then, is supposed to be something rather different from that to which we are accustomed. But this is not all: he comes at last to a monism of a sort, in Creative Evolution and Morality and Religion, saying in the former, e.g., that the UNIVERSE endures, and that “harmony is rather behind us than before. It is due to an identity of impulsion and not to a common aspiration.” (C.E.p.54). How much more is our interest heightened, when in a letter to the R.P. de Tonquedec, he says that the argument by which he establishes the impossibility of the Nought is in no way directed against the existence of a transcendental cause of the world, but, as he has explained (C.E. p.276-9, 298), against the Spinozan (i.e. MONIST) conception of being, and again, that in his works there clearly stands out the idea of a God, creative and free, the generator of both life and matter, and hence a refutation of monism and pantheism in general. (Cp.Chevlr. H.B., p.255n & 270. Eng. trans.). But of this later. The object of this thesis, then, is to endeavour to set in a clearer light the relations of mind and matter, duration and extensity, in Bergson’s philosophy. To this end we shall investigate his works in more or less chronological order, following as far as possible the main divisions of the subject-matter, in order to trace the origin and rise of the dualism on the one hand, and its reconciliation on the other, endeavouring to show that there is a tendency towards a monism of a sort, but a monism which is not ‘static’ like that of Parmenides or of Spinoza, but ‘dynamic’: which indeed resolves into a dualism when we try to express it. To show, then, that is through trying to treat his philosophy, not as the ‘activity’ he says it is, but as a system; not as ‘open’, but as ‘closed’, that folk may see in it a vacillation between Monism and Dualism.