350 THE POPULAR SCIENCE MONTHLY. 



and the action of the forces upon the material particles is likewise an 

 ultimate empirical datura, and therefore inexplicable. Force and 

 matter, though presupposing each other in action, are fundament ally- 

 disparate ; they are essentially distinct, and mutually irreducible en- 

 tities. Matter, as such, is passive, dead ; all motion or life is caused 

 by force ; and the only possible solution of the problems of physiology, 

 no less than of physics and chemistry, consists in the enumeration of 

 the forces acting upon the material particles, and in the exact quan- 

 titative determination of the effects produced by their action. 



This statement of the tenets of the prevailing physical philosophy, 

 to be exact, requires at most two qualifications. In the first place, the 

 recent doctrine of the correlation and mutual convertibility of the physi- 

 cal forces, as a part of the theory of the conservation of energy, has 

 shaken, if not destroyed, the conception of a multiplicity of indepen- 

 dent original forces. And, in the second place, physiologists, like 

 Du Bois-Reymond, recognize force as the invariable concomitant, if 

 not the essential attribute of matter, and assume that to every constant 

 primordial mass belongs a constant primordial quantity of force, so 

 that the problem of physics, chemistry, and physiology, resolves itself 

 into the quantitative determination of the mechanical interactions of 

 material constants primarily endowed with forces acting equally in all 

 directions, or, as they express it, constant central forces. 



I have endeavored, thus far, to show that there are no absolute 

 constants of mass ; that both the hypothesis of corpuscular " atoms " 

 and that of " centres of force " are growths of a confusion of the intel- 

 lect which mistakes conceptual elements of matter for real elements ; 

 that these elements — force and mass, or force and inertia — are not 

 only inseparable, as is conceded by the more thoughtful among modern 

 physicists (or, as they usually, but inaccurately express it, that there 

 is no force without matter, and no matter without force), but that 

 neither of these elements has any reality as such, each of them being 

 simply the conceptual correlate of the other, and thus the condition 

 both of its realization in thought and of its objectivation to sense. 

 The tendency to deal with these elements as separate and separately 

 real entities is so irrepressible, however, that it is necessary to subject 

 them to still further discussion, in order to clear up the prevalent con- 

 fusion in regard to them. 



Newton's original definition of inertia was in terms of force. Ac- 

 cording to him (" Principia," Definitio III.), " there is inherent in mat- 

 ter a force, a power of resistance, in virtue of which every body, as far as 

 in it lies, perseveres in a state of rest, or of uniform rectilinear motion." 

 In the definitions since Newton's time, the term "force" has usually 

 been avoided. Thus Young ("Mechanics," p. 117) defines inertia as 

 " the incapability of matter of altering the state into which it is put 

 by any external cause, whether that state be rest or motion ; " and 

 similarly Whewell ("Mechanics," p. 245), as "the quantity of matter 



