1 86 THE SCIENCE OF LOGIC 



243. METHOD OF CONCOMITANT VARIATIONS. MEASURE 

 MENT. STATISTICS. The methods we have examined so far are 

 often called qualitative methods, as distinct from the present and 

 the next following methods, which are often called quantitative. 

 The former enable us merely to discover the presence, and to 

 some extent the nature, quality, or kind, of the causes and effects 

 investigated ; the latter enable us to take advantage of exact 

 quantitative measurements, to calculate the extent and intensity 

 of the agencies with which we are dealing : an advantage which 

 has contributed enormously to the progress of the physical 

 sciences in modern times. These so-called quantitative methods 

 do not, however, differ in principle from the methods examined ; 

 they, too, are practical plans for eliminating supposed irrelevant 

 elements from the field of investigation. 



The METHOD OF CONCOMITANT VARIATIONS, when applied 

 to phenomena which we can merely observe, may be regarded as 

 an improved application of the method of agreement ; when 

 applied to phenomena which we can control by experiment, it 

 may be considered as a special and more exact application of the 

 method of difference. Its canon is thus formulated by Mill : 



&quot;WHATEVER PHENOMENON VARIES IN ANY MANNER WHEN 

 EVER ANOTHER PHENOMENON VARIES IN SOME PARTICULAR 

 MANNER, IS EITHER A CAUSE OR AN EFFECT OF THAT PHENO 

 MENON, OR IS CONNECTED WITH IT THROUGH SOME FACT OF 

 CAUSATION.&quot; 



In somewhat simpler terms : If two phenomena always vary 

 together, other circumstances remaining the same or varying inde 

 pendently, there is probably a causal connexion between the two 

 phenomena}- Where we have to rely on simple observation, the 

 conclusion as to a causal connexion is only probable, because in 

 such cases we can rarely if ever be sure that all other circum 

 stances do remain the same, or vary independently. For the 

 same reason, simple observation cannot of itself assure us 

 whether the one phenomenon is the cause of the other, or 

 whether both are joint effects of a common cause. For instance, 

 though observation assures us that &quot; the loudness of a clap of 

 thunder varies with the intensity of a flash of lightning,&quot; of itself 



1 MELLONE, op. cit., p, 312. Mr. JOSEPH states it thus: &quot; Nothing is the cause 

 of a phenomenon which varies when it is constant, or is constant when it varies, 

 or varies in no proportionate manner with it&quot; (op. cit., p. 404). Cf. ibid., pp. 

 517-18, where the author points to the difficulty of applying the rule in the social 

 sciences, in which the discovery of reciprocating causes is practically impossible. 



