220 THE PRINCIPLES OF SCIENCE. [CHAP. 



gravity, and a freedom from the corrosive action of acids, 

 we are led to expect that every piece of substance, possess- 

 ing 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 coexistence of 

 qualities ; for the character of the specimens examined 

 alters not with time nor place. 



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

 cipal ground of similarity. When we hear a clock 

 pendulum beat time after time, at equal intervals, 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 stand- 

 ing, may, on like grounds, expect that it will be standing 

 the next evening, and on many succeeding evenings. Even 

 the continuous existence of an object in an unaltered state, 

 or the finding again of that which we have hidden, is but 

 a matter of inference depending on 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 

 the diameter of a circle, with its apex upon the circum- 

 ference, apparently contains a right angle, we may 

 ascertain that all triangles in similar circumstances will 

 contain right angles. This is a case of pure space reason- 

 ing, apart from circumstances of time or quality, and it 

 seems to be governed by different principles of reasoning. 

 I shall endeavour to show, however, that geometrical 

 reasoning differs but in degree from that which applies 

 to other natural relations. 



The Relation of Cause and Effect. 



In a very large part of the scientific investigations 

 which must be considered, we deal with events which 

 follow from previous events, or with existences which 

 succeed existences. Science, indeed, might arise even were 

 material nature a fixed and changeless whole. Endow 

 mind with the power to travel about, and compare part 



