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294 the theologus there was a sort of no-Man's land, which profane persons might enter into and possess. For Aquinas goes on: "Non autem ex parte corporis, nisi secundum habitudinem quam habet corpus ad animam." That is to say, while the anima intellectiva (intellective soul), which issues directly from the hand of God, is the exclusive province of the theologian, the anima sensitiva (sensitive soul), which is propagated in a physical manner, the passions, and the appetites, may be left to the uncowled cultivator of science as not requiring the help of divine inspiration. It was at any rate in this field that the foundations of inductive psychology were laid, and it was to the explanation of the simpler phenomena of sensibility that physical conceptions were first applied. The two greatest thinkers of the seventeenth century were almost simultaneously on the ground.

Descartes is not now remembered by his "Treatise on the Passions" (which was published within a year of Hobbes's work on "Human Nature"), and we only note it here as an early example of experimentalism in Psychology. We are more concerned to observe that his vindication of the immateriality of the thinking principle, and his clear perception of the unity of the mental aggregate, were almost contemporaneous with the "new geometry." We are not, indeed, solicitous with regard to Descartes to justify our thesis—that each advance in Psychology has been caused and conditioned by a corresponding and previous advance in physical science; for the enunciation of the Cartesian principle was less a fact in Psychology than the accomplishment of a stadium in the metaphysical evolution, which made Psychology possible. But, perhaps, it may not seem fanciful to mention that Cavalieri, "the generally reputed father of the new geometry," published in 1635 his Method of Indivisibles (which had been largely anticipated by Kepler), or to connect his leading principle—that a solid is generated out of an infinite number of surfaces placed one above another as their indivisible elements—with the effort to unite into a single substance, itself localized, the endless multiplicity of the mental manifestations. It is, at least, clear, that the application of physics to mind will follow the development of physics; and as physics has not yet advanced beyond the geometrical stage, as the period immediately preceding "Descartes's Meditations" was the epoch of a great geometrical advance, as we now know that in virtue of the consensus (mutual agreement) which governs all social phenomena, all the conceptions of any age are moulded in the same matrix, it seems not wholly imaginary to adduce the psychology of Descartes, who was himself an eminent geometer, as in some degree the result of the dominance of the earliest developed of the sciences.

Emerging from this doubtful region, we pass on to the terra firma (firm ground) of demonstrable fact. Hobbes was rather older than Descartes, but he had the advantage of delaying at least the