OBSERVATION AND EXPERIMENT. 417 



consequents. If those antecedents could not be severed from 

 one another except in thought, or if those consequents never 

 were found apart, it would be impossible for us to distinguish 

 (efc posteriori at least) the real laws, or to assign to any cause 

 its effect, or to any effect its cause. To do so, we must be 

 able to meet with some of the antecedents apart from the rest, 

 and observe what follows from them ; or some of the conse- 

 quents, and observe by what they are preceded. We must, 

 in short, follow the Baconian rule of varying the circumstances. 

 This is, indeed, only the first rule of physical inquiry, and not, 

 as some have thought, the sole rule ; but it is the foundation 

 of all the rest. 



For the purpose of varying the circumstances, we may 

 have recourse (according to a distinction commonly made) 

 either to observation or to experiment ; we may either find an 

 instance in nature, suited to our purposes, or, by an artificial 

 arrangement of circumstances, make one. The value of the 

 instance depends on what it is in itself, not on the mode in 

 which it is obtained : its employment for the purposes of in- 

 duction depends on the same principles in the one case and in 

 the other ; as the uses of money are the same whether it is 

 inherited or acquired. There is, in short, no difference in 

 kind, no real logical distinction, between the two processes of 

 investigation. There are, however, practical distinctions to 

 which it is of considerable importance to advert. 



3. The first and most obvious distinction between 

 Observation and Experiment is, that the latter is an immense 

 extension of the former. It not only enables us to produce 

 a much greater number of variations in the circumstances than 

 nature spontaneously offers, but also, in thousands of cases, to 

 produce the precise sort of variation which we are in want of 

 for discovering the law of the phenomenon ; a service which 

 nature, being constructed on a quite different scheme from 

 that of facilitating our studies, is seldom so friendly as to 

 bestow upon us. For example, in order to ascertain what 

 principle in the atmosphere enables it to sustain life, the 

 variation we require is that a living animal should be immersed 

 VOL. i. 27 



