If body and soul are substances, how can they be dependent on each other in certain of their activities, if they are of opposite natures, how can they affect each other?  How can the incorporeal, unmoved spirit move the animal spirits and receive impulses from them?  The substantiality (reciprocal independence) of body and mind, and their interaction (partial reciprocal dependence), are incompatible, one or the other is illusory and must be abandoned.  The materialists (Hobbes) sacrifice the independence of mind, the idealists (Berkeley, Leibnitz), the independence of matter, the occasionalists, the interaction of the two.  This forms the advance of the last beyond Descartes, who either naively maintains that, in spite of the contrariety of material and mental substances, an exchange of effects takes place between them as an empirical fact, or, when he realizes the difficulty of the anthropological problem,—­how is the union of the two substances in man possible,—­ascribes the interaction of body and mind, together with the union of the two, to the power of God, and by this abandonment of the attempt at a natural explanation, opens up the occasionalistic way of escape.  Further, in his more detailed description of the intercourse between body and mind Descartes had been guilty of direct violations of his laws of natural philosophy.  If the quantity of motion is declared to be invariable and a change in its direction is attributed to mechanical causes alone, we must not ascribe to the soul the power to move the pineal gland, even in the gentlest way, nor to control the direction of the animal spirits.  These inconsistencies also are removed by the occasionalistic thesis.

The question concerning the substantiality of mind and matter in relation to God, is involved from the very beginning in this latter problem, “How is the appearance of interaction between the two to be explained without detriment to their substantiality in relation to each other?” The denial of the reciprocal dependence of matter and spirit leads to sharper accentuation of their common dependence upon God.  Thus occasionalism forms the transition to the pantheism of Spinoza, Geulincx emphasizing the non-substantiality of spirits, and Malebranche the non-substantiality of bodies, while Spinoza combines and intensifies both.  And yet history was not obliging enough to carry out this convenient and agreeable scheme of development with chronological accuracy, for she had Spinoza complete his pantheism before Malebranche had prepared the way.  The relation which was noted in the case of Bruno and Campanella is here repeated:  the earlier thinker assumes the more advanced position, while the later one seems backward in comparison; and that which, viewed from the standpoint of the question itself, may be considered a transition link, is historically to be taken as a reaction against the excessive prosecution of a line of thought which, up to a certain point, had been followed by the one who now shrinks back from its