252 THE PRINCIPLES OF SCIENCE. 



reasoning, yet a diversity arises in their application. 

 Similarity of conditions between the events from which 

 we argue, and those to which we argue, must always be 

 the ground of inference; but this similarity may have 

 regard either to time or place, or the simple logical 

 combination of events, or to any conceivable junction of 

 circumstances involving quality, time, and place. Having 

 met with many pieces of substance possessing ductility, 

 and a bright yellow colour, and having discovered, by 

 perfect induction, that they all possess in addition a high 

 specific gravity, and a freedom from the corrosive action 

 of acids, we are led to expect that every piece of substance, 

 possessing like ductility, and a similar yellow colour, will 

 have an equally high specific gravity, and a like freedom 

 from corrosion by acids. This is a case of the co-existence 

 of qualities ; for the character of the specimens examined 

 alters not with time or place. 



In a second class of cases, time will enter as a prin- 

 cipal ground of similarity. When we hear a clock 

 pendulum beat moment after moment, at equal in- 

 tervals, and with a uniform sound, we confidently expect 

 that the stroke will continue to be repeated uniformly. 

 A comet having appeared several times at nearly equal 

 intervals, we infer that it will probably appear again 

 at the end of another like interval. A man who has 

 returned home evening after evening for many years, 

 and found his house standing, may, on like grounds, 

 expect that it will be standing the next evening, and on 

 many succeeding evenings. Even the continuous exist- 

 ence of an object in an unaltered state, or the finding 

 again of that which we have hidden, is but a matter of 

 inference to be decided by experience. 



A still larger and more complex class of cases involves 

 the relations of space, in addition to those of time and 

 quality. Having observed that every triangle drawn upon 



