4 o HERMANN VON HELMHOLTZ 



are unchangeable, and which therefore must, in all cases 

 where the exterior conditions are the same, produce the same 

 invariable effects. The final aim of the theoretic natural 

 sciences is therefore to discover the ultimate and unchange- 

 able causes of natural phenomena. Whether all the processes 

 of nature be actually referable to such, or whether nature is 

 capable of being completely comprehended, or whether changes 

 occur which are not subject to the laws of necessary causation, 

 but spring from spontaneity or freedom, this is not the place 

 to decide ; it is at all events clear that the science whose 

 object it is to comprehend nature must proceed from the 

 assumption that it is comprehensible, and in accordance with 

 this assumption investigate and conclude until, perhaps, she 

 is at length admonished by irrefragable facts that there are 

 limits beyond which she cannot proceed. 



' Science regards the phenomena of the exterior world ac- 

 cording to two processes of abstraction; in the first place it 

 looks upon them as simple existences, without regard to their 

 action upon our organs of sense or upon each other; in this 

 aspect they are termed matter. The existence of matter in 

 itself is to us something tranquil and devoid of action : in it 

 we distinguish merely the relations of space and of quantity 

 (mass), which is assumed to be eternally unchangeable. To 

 matter, thus regarded, we must not ascribe qualitative differ- 

 ences, for when we speak of different kinds of matter we refer 

 to differences of action, that is, to differences in the forces of 

 matter. Matter in itself can therefore partake of one change 

 only, a change which has reference to space, that is, motion. 

 Natural objects are not, however, thus passive ; in fact we 

 come to a knowledge of their existence solely from their 

 actions upon our organs of sense, and infer from these actions 

 a something which acts. When, therefore, we wish to make 

 actual application of our idea of matter, we can only do it by 

 means of a second abstraction, and ascribe to it properties 

 which in the first case were excluded from our idea, namely, 

 the capability of producing effects, or, in other words, of 

 exerting force. It is evident that in the application of the 

 ideas of matter and force to nature the two former should 

 never be separated : a mass of pure matter would as far as 

 we and nature are concerned be a nullity, inasmuch as no 



