METHOD OF DISCOVERING CAUSAL LAWS 177 



ment, and in all similar sets of conditions. Further experiment is 

 hardly needed to convince us that it is the resistance of the 

 medium that necessitates the retardation : not necessarily atmo 

 spheric resistance, but the resistance of any other gases that 

 we might substitute for the atmosphere. Yet, strictly speaking, 

 the single experiment with the atmosphere assures us only that 

 the air is a cause of retardation in the fall of the feather. 1 But 

 granted, now, that we have made, if necessary, such experiments 

 with other gaseous media, and convinced ourselves that in the 

 circumstances of those experiments, and therefore in all similar sets 

 of conditions, the resistance of a gaseous medium is the only possible 

 cause of the phenomenon, can we further infer, from such single 

 experiments, that this is the only possible cause of the phenome 

 non, absolutely and universally, i.e. in any and every conceivable 

 place, time, state or condition of things, in the universe ? No ; the 

 method of single difference does not by itself really guarantee 

 this further inference. What it brings to light as the &quot;only 

 possible cause &quot; may perhaps be not yet sufficiently analysed, 

 may perhaps contain &quot; irrelevant matter &quot; plus &quot; the only possible 

 cause &quot; ; so that this latter may perhaps be forthcoming else 

 where, in quite a different vehicle, other than that in which it was 

 embodied in any of our experiments. Hence the method of 

 single difference proves a cause ; but of itself it does not strictly 

 prove that this is the only possible cause. But the latter is the 

 only kind of cause that excludes plurality ; there may be a 

 plurality of causes of the former kind. Therefore, the present 

 method does not of itself completely exclude the uncertainty which 

 arises from the possibility of a plurality of causes . In other words, 

 the method of single difference does not of itself establish with 

 certitude a reciprocating causal relation. 



Another illustration of the present rule and its limitations, is 

 afforded by experimental inquiry into the conditions for the pro 

 pagation of sound. 2 It is supposed that the presence of some 

 elastic medium, such as air (or any other gas), between the ear and 

 the sounding body, is an indispensable condition for the propaga 

 tion of sound. This hypothesis is tested by ringing a bell in the 



1 By substituting successively, for air, other gaseous media, having presumably 

 only RESISTING POWER in common, as a factor relevant to retardation of the feather, 

 and finding the retardation take place in every case, we satisfy ourselves, as far as we 

 can, by this application of the Method of Agreement, that it is resistance that causes 

 retardation. 



2 Cf. JOSEPH, op. cit,, p. 442. 



VOL. II. 12 



