i.J INTRODUCTION. 



can never have examined and registered possible exist- 

 ences so thoroughly as to be sure that no new ones will 

 occur and frustrate our calculations. The same outward 

 appearances may cover any amount of hidden differences 

 which we have not yet suspected. To the variety of sub- 

 stances and powers diffused through nature at its creation, 

 we should not suppose that our brief experience can assign 

 a limit, and the necessary imperfection of our knowledge 

 must be ever borne in mind. 



Yet there is much to give us confidence in Science. The 

 wider our experience, the more minute our examination of 

 the globe, the greater the accumulation of well-reasoned 

 knowledge, the fewer in all probability will be the failures 

 of inference compared with the successes. Exceptions 

 to the prevalence of Law are gradually reduced to Law 

 themselves. Certain deep similarities have been detected 

 among the objects around us, and have never yet been 

 found wanting. As the means of examining distant parts 

 of the universe have been acquired, those similarities have 

 been traced there as here. Other worlds and stellar 

 systems may be almost incomprehensively different from 

 ours in magnitude, condition and disposition of parts, and 

 yet we detect there the same elements of which our own 

 limbs are composed. The same natural laws can be 

 detected in operation in every part of the universe within 

 the scope of our instruments ; and doubtless these laws are 

 obeyed irrespective of distance, time, and circumstance. 



It is the prerogative of Intellect to discover what is uni- 

 form and unchanging in the phenomena around us. So 

 far as object is different from object, knowledge is useless 

 and inference impossible. But so far as object resembles 

 object, we can pass from one to the other. In proportion 

 as resemblance is deeper and more general, the com- 

 manding powers of knowledge become more wonderful. 

 Identity in one or other of its phases is thus always 

 the bridge by which we pass in inference from case to 

 case ; and it is my purpose in this treatise to trace out the 

 various forms in which the one same process of reasoning- 

 presents itself in the ever-growing achievements of Scientific 

 Method. 



B 2 



