186 GENERAL CONDITIONS OF SCIENTIFIC RESEARCH. 



From this order we have further acquired the notion that 

 each simple science is governed not only by its own laws 

 and principles, but also by the more general of those of all 

 the other sciences which precede it in the series ; and con- 

 sequently that some of the characters and modes of action 

 of one force can be traced in the phenomena of all the 

 succeeding forces. ( It seems quite certain that electricity 

 in motion is heat.' l 



It must not be forgotten, however, that discoveries are 

 made both by induction and deduction. The human mind, 

 possessing a knowledge of particular scientific truths, can, 

 by appropriate and different methods, advance by their aid 

 to the discovery of others ; it can by induction discover 

 their cause or principle, or by deduction determine their 

 effects ; it can also ascertain their coincidences. In induc- 

 tion, therefore, the discovery of a truth is dependent upon 

 the previous knowledge of particular instances of a less 

 general kind ; but in deduction the discovery of particular 

 instances arises from previous knowledge of the principle 

 which governs them. The simpler sciences are in this 

 way developed by the aid of the more complex ones simul- 

 taneously with the development of the latter by the aid 

 of the former. The chronological order of discovery is 

 therefore of a dual character, and is of a reverse kind in 

 the two cases. It follows also from this, and is confirmed 

 by a great variety of facts, that the discovery of new truths 

 in a science is not only dependent upon the prior develop- 

 ment of certain parts of the simpler sciences, but also of 

 the more complex ones, and that all the sciences act and 

 react upon each other to further the progress of discovery. 

 This shows that the principle of action and reaction 

 operates even in the development of new scientific truths. 

 In actual science we also find that whilst under one set of 



1 Thomson, Electrostatics, p. 224. 



