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LXXVIII. Letter to Henry Lord Brougham, F.R.S., $c, 

 containing Remarks on certain Statements in his Lives of 

 Black, Watt and Cavendish. By the Rev. William Vernon 

 Harcourt, F.R.S. Src. 



[Continued from p. 131.] 

 THHERE are few things more remarkable in scientific hi- 

 * story than the manner in which Newton may be ob- 

 served to have dealt with the conjectural part of philosophy. 

 He never speaks of hypothetical speculation but in terms im- 

 plying somewhat of disdain. And yet in all his works, from 

 the announcement to the Royal Society of his first discoveries 

 respecting light to the last revision of the Optics and Prin- 

 cipia, an hypothesis of the highest generality holds a conspi- 

 cuous place. 



This apparent inconsistence is however easily explained : 

 he doubtless was deeply impressed with the error into which 

 his predecessor Descartes had fallen, in building a system of 

 philosophy on superficial analogies and precarious conjec- 

 tures, and looked with some dissatisfaction at the pretension 

 of his cotemporary Hook to set aside the inductive analysis 

 of light, on the faith of a conjectural standard of his own. 

 With Newton the imagining hypotheses was but as child's- 

 play compared with the labour and importance of those severe 

 and sure processes, inductive and deductive, to which he had 

 devoted all the efforts of his mind. He held cheap the exer- 

 cise of that great faculty of imagination from which the inex- 

 haustible riches of his philosophical invention flowed with 

 spontaneous facility. But though he laid no stress on what 

 he called his "guesses," no man's mind seems ever to have 

 been more continually, as it were, upon the guess ; and no one 

 ever gave so eminent and instructive an example of steady 

 persistence in that conjectural habit of mind. " To show," 

 says Newton, " that I do not take gravity for an essential pro- 

 perty of bodies, I have added one question concerning its 

 cause, choosing to propose it by way of question because I am 

 not yet satisfied about it for want of experiments*." After 

 having himself achieved by a vigorous induction the most ex- 

 tensive generalisation to which the human intellect has ever 

 attained, he still saw, in a stronger light than any one, reasons 

 for doubting whether the law at which he had arrived was so 

 simple and conformable to the rest of nature as to preclude 

 our tracing it to some more general cause. The ascertained 

 rule of gravitation he used but as a stepping-stone on which 

 he might safely tread in advancing towards the great end of 



* Advertisement to Optics, 1717- 



