er somewhat out of the main lines of communication. Hobbes was, moreover, a traveller, had lived much on the Continent, and had possibly met Galileo at Pisa. It was under the influence of these two men, or rather of the methods they represented—Descartes and mathematics, Galileo and the laws of motion—that Hobbes proceeded to work out his philosophy. In the language of a distinguished professor, to whom we look for an exhaustive account of Hobbes's relations to the science of his time, "he set about reducing all his thoughts into the unity of a system, whose central idea was this of motion, and whose guiding principles were those of mathematical deduction."[1] "His great postulate," says the same writer, "is motion or mutation,"[2] and he makes copious use of it within the sphere to which Aquinas banished the experimental psychologist, and a little beyond. His explanation of sensation is wholly mechanical. The crass materialism with which he set out may have had something to do with his trenchant rejection of the audible, visible, and intelligible species of the Schoolmen, but the hypothesis which replaced them betrays its own origin. "The apparition of light," he says, "is really nothing but motion within."[3] This thesis is more elaborately developed in a passage which we quote at length, as it appears to contain an anticipation of the undulatory theory of light and heat:
"From all lucid, shining, and illuminate bodies, there is a motion produced to the eye, and, through the eye, to the optic nerve, and so into the brain, by which that apparition of light and color is effected.... First, it is evident that the fire... worketh by motion equally every way.... And further, that that motion, whereby the fire worketh, is dilation, and contraction of itself alternately... is manifest also by experience. From such motion in the fire must needs arise a rejection or casting from itself of that part of the medium which is contiguous to it, whereby that part also rejecteth the next, and so successively one part beateth back another to the very eye," and so from the eye to the optic nerve, and from that to the brain.[4]
This postulate of motion, applied in this thorough-going manner, led Hobbes to a great discovery in the psychology of sensation. He clearly demonstrated that the secondary qualities of body are purely subjective, and his language is almost strong enough to lead us to believe that he would have gone a long way with Berkeley. For he claims to have proved that "as in vision, so also in conceptions that arise from the other senses, the subject of their inherence is not the object but the sentient." If the word "conceptions" be interpreted according to a definition previously laid down in the same treatise, in which the "images produced by things" are described as conceptions, imaginations, ideas, knowledge, it should seem that he might have applied the analysis to the primary qualities as well, had the two sets of properties been as sharply contrasted as now, instead of being first dis-
