the species to the genus; philosophical knowledge of facts under the relation of cause and effect; a cheerful and communicative temper disposing us to notice the similarities and contrasts of things, that we may be able to illustrate the one by the other; a quiet conscience; a condition free from anxieties; sound health, and above all (as far as relates to passive remembrance) a healthy digestion; these are the best, these are the only Arts of Memory. CHAPTER VIII The system of Dualism introduced by Des Cartes--Refined first by Spinoza and afterwards by Leibnitz into the doctrine of Harmonia praestabilita--Hylozoism--Materialism--None of these systems, or any possible theory of association, supplies or supersedes a theory of perception, or explains the formation of the associable. To the best of my knowledge Des Cartes was the first philosopher who introduced the absolute and essential heterogenity of the soul as intelligence, and the body as matter. The assumption, and the form of speaking have remained, though the denial of all other properties to matter but that of extension, on which denial the whole system of Dualism is grounded, has been long exploded. For since impenetrability is intelligible only as a mode of resistance; its admission places the essence of matter in an act or power, which it possesses in common with spirit; and body and spirit are therefore no longer absolutely heterogeneous, but may without any absurdity be supposed to be different modes, or degrees in perfection, of a common substratum. To this possibility, however, it was not the fashion to advert. The soul was a thinking substance, and body a space-filling substance. Yet the apparent action of each on the other pressed heavy on the philosopher on the one hand; and no less heavily on the other hand pressed the evident truth, that the law of causality holds only between homogeneous things, that is, things having some common property; and cannot extend from one world into another, its contrary. A close analysis evinced it to be no less absurd than the question whether a man’s affection for his wife lay North-east, or South-west of the love he bore towards his child. Leibnitz’s doctrine of a pre-established harmony; which he certainly borrowed from Spinoza, who had himself taken the hint from Des Cartes’s animal machines, was in its common interpretation too strange to survive the inventor--too repugnant to our common sense; which is not indeed entitled to a judicial voice in the courts of scientific philosophy; but whose whispers still exert a strong secret influence. Even Wolf, the admirer and illustrious systematizer of the Leibnitzian doctrine, contents himself with defending the possibility of the idea, but does not adopt it as a part of the edifice.