668 PSYCHOLOGY. 



of the processes in Nature. Whether all processes be really ascribable 

 to such causes, whether, in other words, nature he completely intelUyible. 

 or whether there be changes which would elude the law of a necessary 

 causality, and fall into a realm of spontaneity or freedom, is not here 

 the place to determine ; but at any rate it is clear that the Science 

 whose aim it is to make nature appear intelligible [die Natur zu 

 begreifen] must start with the assumption of her intelligibility, and 

 draw consequences in conformity with this assumption, until irrefu- 

 table facts show the limitations of this method. , . . The postulate that 

 natural phenomena must be reduced to changeless ultimate causes next 

 shapes itself so that forces nnchanyed hy time must be found to be 

 these causes. Now in Science we have already found portions of mat- 

 ter with changeless forces (indestructible qualities), and called them 

 (chemical) elements. If, then, we imagine the world composed of ele- 

 ments with inalterable qualities, the only changes that can remai.^ 

 possible in such a world ai'e spatial changes, i.e. movements, and the 

 only outer relations which can modify the action of the forces are 

 spatial too . or, in other words, the forces are motor forces dependent 

 for their effect only on spatial relations. More exactly still : The phe- 

 nomena of nature must be reduced to [zurHckgefiihrt, conceived as, 

 classed as] motions of material points with inalterable motor forces 

 acting according to space-relations alone. . . . But points have no 

 mutual space-relations except their distance, . . . and a motor force 

 which they exert upon each other can cause nothing but a change of 

 distance— i.e. be an attractive or a repulsive force. . . . And its inten- 

 sity can only depend on distance. So that at last the task of Physics 

 resolves itself into this, to refer phenomena to inalterable attractive 

 and I'epulsive forces whose intensity varies with distance. The solu- 

 tion of this task would at the same time be the condition of Nature's 

 complete intelligibility."* 



The subjective interest leading to tlie assumption could 

 not be more candidly expressed. What makes the assump- 

 tion ' scientific ' and not merely poetic, what makes a Helm- 

 holtz and his kin discoverers, is that the things of Nature 

 turn out to act as if they luere of the kind assumed. They 

 behave as such mere drawing and driving atoms would be- 

 have ; and so far as they have been distinctly enough trans- 

 lated into molecular terms to test the point, so far a certain 

 fantastically ideal object, namely, the mathematical sum 

 containing their mutual distances and velocities, is found to 

 be constant throughout all their movements. This sum is 

 called the total energy of the molecules considered. Its con- 



* Die Erhaltuiig der Kraft (1847), pp. 3-6. 



