THE DEVELOPMENT OF PSYCHOLOGY. 299 



disputed discovery of the acquired nature of our perceptions of dis- 

 tance 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, E'ewton'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 1*709, 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 incorpo- 

 rated 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 NcAvton," ^ and 

 appropriating from the "Principia" the hypothesis of vibrations by 

 which he explained sensation : 



"Mj chief design in the following chapter is, briefly, to explain, establish, 

 and apply the doctrines of vilrations and association. The first of these doc- 

 trines 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 as- 

 sociation 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 associutions as their effect, and association point to 

 vibrations as its cause." ^ 



It may seem somewhat bold in Hartley, whose name has almost 

 passed into a by-word as that of an hypothesis-maker, to shelter him- 

 self under the aegis 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." ^ 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 suflicient to cope with the accumulated multitude of as- 

 certained facts, and when some comprehensive hypothesis is required 

 which shall connect the empirical generalizations of one science with 

 tlie ultimate laws of Nature and tlie principles of all the sciences. 

 Newton's force of gravity and Hartley's theory of vibrations were 

 such hypotheses. But, besides the intrinsic difiierence 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 

 ' " Observations," ch. i. ^ " Ibid.," ch. i. ^ " Principles of Science," ii., 228. 



