1 88 THE SCIENCE OF LOGIC 



which vary in roughness, and measuring the rate at which in 

 crease of friction (the supposed cause) gradually retards the 

 motion. 



From those examples it will be seen that the present method 

 is applicable to a class of experimental cases in which the method 

 of difference proper cannot be employed : viz. cases in which the 

 supposed causal agency or agencies are what are called &quot; PER 

 MANENT CAUSES,&quot; i.e. of such a kind that they cannot be totally 

 eliminated from any experiment : friction, gravity, heat, electric 

 and magnetic influences, etc. Two instances in which the phe 

 nomenon is present in a greater and in a lesser degree may be 

 taken as representing the &quot; positive &quot; and the &quot; negative &quot; instance 

 respectively (the presence of a phenomenon in a lesser degree being 

 really the absence of a greater degree of that phenomenon) ; and 

 in this way the present method may be regarded as a modifica 

 tion of the method, of difference. It perfects the knowledge 

 obtained by the latter method, in so far as this knowledge bears 

 upon the quantitative proportion between cause and effect. 



Numerous instruments for measuring are based upon the as 

 certained concomitant variations of certain natural causes and 

 effects. The thermometer, for instance, depends upon the con 

 comitant variation of heat and the expansion of mercury (or 

 certain substitutes, such as alcohol) in volume ; and the baro 

 meter on the concomitant variation of atmospheric weight or 

 pressure and the height of a column of mercury (or other fluids) 

 supported by that pressure. 



While the method of concomitant variations is more exact 

 than the method of difference, by enabling us to measure the 

 quantitative proportion between cause and effect within the limits 

 of observation and experiment?- at the same time any inference that 

 the variation will continue at a regular rate beyond observed limits 

 is, for the most part, hazardous and unreliable : inasmuch as, under 

 changed conditions, other agencies may become operative. 2 

 When the concomitant variations of physical phenomena can be 

 exactly measured within certain limits, and when we are sure that 

 all other circumstances are irrelevant, the variations may be such 

 as to enable us to conclude that the one phenomenon is the total 

 and indispensable cause of the other within these limits, and that 

 therefore it will continue such beyond these limits. Hence, al 

 though we can have no experience of perpetual rectilinear motion, 



l Cf. VENN, Empirical Logic, pp. 419-20. Cf. JOSEPH, op. cit., 463. 



