218 INDUCTION. 



the circumstances. This is, indeed, only the Jirst rule of physical inqui- 

 ry, and not, as some have thought, the sole rule ; but it is the founda- 

 tion of all the rest. 



Foi- 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 upon what it is in itself, not upon 

 the mode in which it is obtained : its employment for the puqioses 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 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, moreover, 

 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 ser- 

 vice which nature, being constrvicted 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 atmos- 

 phere enables it to sustain life, the variation we require is that a living 

 animal should be immersed in each component element of the atmos- 

 phere separately. But nature does not supply either oxygen or azote 

 in a separate state. We are indebted to artificial experiment for our 

 knowledge that it is the former, and not the latter, which supports 

 respiration ; and even for our knowledge of the very existence of the 

 two ingredients. 



Thus far the advantage of experimentation over simple obsez'vation 

 is universally recognized : all are aware that it enables us to obtain 

 innumerable combinations of circumstances which are not to be found 

 in nature, and so add to nature's experiments a multitude of experi- 

 ments of our own. But there is another superiority (or, as Bacon 

 would have expressed it, another prerogative), of instances artificially 

 obtained over spontaneous instances — of our own experiments over 

 even the same experiments when made by nature — which is not of less 

 importance, and which is far from being felt and acknowledged in the 

 same degree. 



"When we can produce a phenomenon artificially, we can take it, as 

 it were, home with us, and observe it in the midst of circumstances 

 with which in all other respects we are accurately acquainted. If we 

 desire to know what are the effects of the cause A, and are able to 

 produce A by any means at our disposal, we can generally determine 

 at our own discretion, so far as js compatible with the nature of the 

 phenomenon A, the whole of the circumstances which shall be present 

 along with it : and thus, knowing exactly the simultaneous state of every- 

 thing else which is within the reach of A's influence, we have only to 

 observe what alteration is made in that state by the prissence of A. 



For example, by the electrical machine we can produce in the midst 



