198 CORRELATION OF PHYSICAL FOECE8. 



its turn, should the science continue to progress, either gives place 

 to a more simple and wider generalisation, or be comes, by the re- 

 moval of objections, established as a law. Even in this more 

 advanced stage, words importing theory must be used, but 

 phenomena are now intelligible and connected, though express- 

 ed by varied forms of speech. 



To think on nature is to theorise ; and difficult it is not 

 to be led on by the continuities of natural phenomena to the- 

 ories which appear forced and unintelligible to those who 

 have not pursued the same path of thought : which, more- 

 over, if allowed to gain an undue influence, seduce us from 

 that truth which is the sole object of our pursuit. 



Where to draw the line where to say thus far we may 

 go, and no farther, in any particular class of analogies or re- 

 lations which Nature presents to us ; how far to follow the 

 progressive indications of thought, and where to resist its al- 

 lurements is a question of degree which must depend upon 

 the judgment of each individual or of each class of thinkers ; 

 yet it is consolatory that thought is seldom expended in vain. 



I have throughout endeavoured to discard the hypotheses 

 of subtle or occult entities ; if in this endeavour some of my 

 views have been adopted upon insufficient data, I still hope 

 that this essay will not prove valueless. 



The conviction that the so-called imponderables are modes 

 of motion, will, at all events, lead the observer of natural 

 phenomena to look for changes in these affections, wherever 

 the intimate structure of matter is changed ; and, conversely, 

 to seek for changes in matter, either temporary or permanent, 

 whenever it is affected by these forces. I believe he will 

 seldom do this in vain. It was not until I had long reflected 

 on the subject, that I ventured to publish my views : their 

 publication may induce others to think on their subject-mat- 

 ter. They are not put forward with the same objects, nor do 

 they aim at the same elaboration of detail, as memoirs on 

 newly-discovered physical facts : they purport to be a method 



