disputed discovery of the acquired nature of our perceptions of distance we may pass over with the remark that, if the genesis of it could be traced, it would probably be found to have derived its impulse from that "century of inventions" which witnessed Snell's discovery of the law of refraction in 1624, Newton's discoveries in the composition of light in 1674, Huyghens's proof of the polarization of light about 1692, and the explanation of the structure of the eye by Petit in 1700. The conjunction will seem more than a coincidence if it is added that Berkeley's "Theory of Vision," which appeared in 1709, was preceded by Newton's "Optics," in 1705.
The next great advance of Psychology combined, in principle, the advances made by both Hobbes and Locke. As Hobbes had incorporated the conceptions of physical science, and Locke had adopted its methods, we find Hartley professing to follow the "method of analysis and synthesis recommended and followed by Sir Isaac Newton,"[1] and appropriating from the "Principia" the hypothesis of vibrations by which he explained sensation:
"My chief design in the following chapter is, briefly, to explain, establish, and apply the doctrines of vibrations and association. The first of these doctrines is taken from the hints concerning the performance of sensation and motion, which Sir Isaac Newton has given at the end of his 'Principia,' and in the questions annexed to his 'Optics;' the last from what Mr. Locke and other ingenious persons since his time have delivered concerning the influence of association over our opinions and affections, and its use in explaining those things in an accurate and precise way, which are commonly referred to the power of habit and custom, in a general and indeterminate one.... One may expect that vibrations should infer associations as their effect, and association point to vibrations as its cause."[2]
It may seem somewhat bold in Hartley, whose name has almost passed into a by-word as that of an hypothesis-maker, to shelter himself under the ægis of Newton, who declarad—"hypotheses non fingo." But, as is observed by Prof. Stanley Jevons, "the greater part of the 'Principia' is purely hypothetical, endless varieties of causes and laws being imagined which have no counterpart in Nature."[3] Psychology had reached in Hartley's time, as Natural Philosophy in Newton's time, the stage when the mere generalization of observed uniformities is no longer sufficient to cope with the accumulated multitude of ascertained facts, and when some comprehensive hypothesis is required which shall connect the empirical generalizations of one science with the ultimate laws of Nature and the principles of all the sciences. Newton's force of gravity and Hartley's theory of vibrations were such hypotheses. But, besides the intrinsic difference between them residing in the fact that the one could be proved, and the other, at best, only made probable, there was the further contrariety, which explains their very different success, that the Newtonian conception
