OBSERVATION AND EXPERIMENT. 441 



only the first rule of physical inquiry, and not, as 

 some have thought, the sole rule ; but it is the foun- 

 dation of all the rest. 



For the purpose of varying the circumstances, we 

 may have recourse (according to a distinction com- 

 monly 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 cir- 

 cumstances, make one. The value of the instance 

 depends upon what it is in itself, not upon the mode 

 in which it is obtained : its employment for the pur- 

 poses of induction depends upon 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 distinc- 

 tions 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 sponta- 

 neously offers, but, moreover, in thousands of cases, 

 to produce the precise sort of variation which we 

 are in want of for discovering the law of the pheno- 

 menon; 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 in each component element of the atmo- 



