THE PHILOSOPHICAL WORKS. 61 



to certain rules. Between arranging stones so as to form a 

 house, and arranging particles so as to produce whiteness, there 

 is no difference but that of scale. So in other cases. The 

 difference of scale once set aside, it seemed to follow that the 

 knowledge of the Form would in all cases lead to great practical 

 results. 



Thus far of the end which the new philosophy proposes to 

 itself, and of the method which it must employ. The next 

 question relates to the mode of procuring and arranging the 

 materials on which this method is to work. In this part of the 

 subject we again perceive the influence of Bacon's opinion 

 touching the limitedness of Nature. No one acquainted with 

 the history of natural philosophy would think it possible to 

 form a collection of all the facts which are to be the materials 

 on which any science is to operate, antecedently to the formation 

 of the science itself. 



In the first place, the observations necessary in order to the 

 recognition of these facts would never have been made except 

 under the guidance of some preconceived idea as to the subject 

 of observation ; and in the second, the statement which embodies 

 the result of observation always involves some portion of theory. 

 According to the common use of language, it is a fact and not a 

 theory that in ordinary refraction the sine of the angle of in- 

 cidence is to the sine of the angle of refraction in a given ratio. 

 But the observations on which this statement is based, and the 

 statement itself, presuppose the recognition of a portion of the 

 theory of light, namely that light is propagated in straight lines 

 in other words, they presuppose the conception of a ray. Nor 

 would these observations have been made but for the idea in the 

 mind of the observers that the magnitude of the angle of refrac- 

 tion depends on that of the angle of incidence. 



As we advance farther in any science, what we call facts in- 

 volve more and more of theory. Thus it is a fact that the 

 tangent of the angle of polarisation is equal to the index of re- 

 fraction. But no one could have made the observations which 

 prove it, or have stated their result in words, without a distinct 

 conception, first of the law of refraction, and secondly of the 

 distinguishing character of polarised light. 



The history of science and the nature of the case concur in 

 showing that observation and theory must go on together; it 

 is impossible that the one can be completed before the other 



