1 78 THE SCIENCE OF LOGIC 



receiver of an air-pump, first before exhausting, then after ex 

 hausting, the receiver. In the former (positive) instance the sound 

 is heard, in the latter (negative) instance it is not. From this 

 we conclude that the presence of air is a sufficient condition for 

 the propagation of sound ; not, however, that it is itself an indis 

 pensable condition, but only that it contains an indispensable 

 condition supposed to be elasticity. This latter supposition 

 we may confirm by applying the method of agreement to a 

 number of positive instances, each containing a different elastic 

 medium. But even when this stage has been reached we cannot, 

 strictly speaking, be certain that elasticity is the only relevant 

 factor common to the media employed in the positive instances ; 

 nor, therefore, that it is a really indispensable factor in those 

 instances. Nor further, even were we convinced that elasticity 

 was one of the indispensable factors in the conditions of our experi 

 ments^ could we, strictly speaking, conclude that an elastic medium 

 is indispensable to the propagation of sound in any and every con 

 ceivable set of circumstances. For it might still be objected that 

 perhaps in some totally different set of circumstances a medium 

 with some other property instead of elasticity might be capable of 

 propagating sound. Such an alternative possibility we might be 

 disposed to regard as far-fetched ; but the only way of disproving 

 any such suggestion would be by obtaining a negative experiment 

 in which elasticity would be absent and the other supposed pro 

 perty present, arid in which sound would not be propagated. It will 

 be seen that the experiments throughout this example do not 

 purport to prove that elasticity is the only indispensable condition 

 of a sound-propagating medium, but only that it is an indispens 

 able condition. There might be elastic media which would not 

 propagate sound, owing to the absence of other indispensable con 

 ditions for such propagation. The hypothetical ether, supposed 

 to be the propagating medium of radiant heat, light, electric and 

 magnetic influences, may, perhaps, be incapable of transmitting 

 sound, even though it be elastic. 



The method of difference has been stated in the following terms by Mill : 

 &quot; If an instance in which the phenomenon under investigation occurs, and 

 an instance in which it does not occur, have every circumstance in common 

 save one, that one occurring only in the former, the circumstance in which 

 alone the two instances differ, is the effect, or the cause, or an indispensable 

 part of the cause, of the phenomenon&quot; 1 



1 Logic, III., viii., 2. 



