176 THE SCIENCE OF LOGIC 



that two instances offered by nature are of this sort. It is 

 only when we have thoroughly analysed all the surroundings of 

 the phenomenon, when we know all of them, when we can con 

 trol, modify, introduce, or remove, each of them separately, with 

 out disturbing the others, that we can be sure our two instances 

 have everything else in common, and differ only in the one essential 

 respect. This latter condition, we may remark, is why we call 

 this rule the &quot; method of difference &quot;. 



In applying the rule experimentally it does not matter which 

 of the two instances, the positive or the negative, comes first in 

 order of time. Occasionally, we may find it more satisfactory 

 or, rather, less unsatisfactory to produce the two instances simul 

 taneously. &quot; E.g. to try the effect of a certain manure on a 

 wheat crop, you would not try it one year and compare the 

 result with the year before, for the weather might be different. 

 You would take two fields exactly alike and try the manure in 

 one of them and not the other.&quot; * 



For the most part, however, the instances are procured suc 

 cessively. The well-known coin and feather experiment will 

 afford a simple illustration. It is supposed that the greater 

 resistance of the air to the relatively larger volume of the lighter 

 sorts of bodies is the reason why these fall more slowly than 

 bodies of the heavier sort. This is the hypothesis for verifica 

 tion. To test it we contrive an experiment by means of the air- 

 pump. Before exhausting the receiver we take the coin and 

 feather and let them fall within the receiver : they fall in unequal 

 times (as they would outside the receiver) : the supposed cause 

 and the effect are both present, the resistance of the air and the 

 retardation of the fall of the feather. This is the positive in 

 stance. We next eliminate the supposed cause by exhausting 

 the receiver of air : we let the coin and feather fall in the ex 

 hausted receiver, and we observe that the effect has disappeared ; 

 the feather is not retarded, it falls as quickly as the coin. This 

 is the negative instance. Since, then, so far as we know, there 

 was no change in the circumstances of the falling coin and 

 feather, except the removal of the air, we conclude that the air 

 contained (among other things, assumed to be irrelevant) that 

 element (namely, resistance) which is the necessitating and only 

 possible cause of the phenomenon in the conditions of our experi- 



1 Palaestra Logica, p. no, 341. Cf. MELLONE, op. cit., p. 303 ; and JOSEPH, 

 op. cit., p. 519, for the limitations of such an experiment. 



