710 SCIENCE AND HYPOTHESIS 



a tautology ? Not at all. It means that the different things to which 

 we give the name of energy are connected by a true relationship; it 

 affirms between them a real relation. But then, if this principle haa 

 a meaning, it may be false ; it may be that we have no right to extend 

 indefinitely its applications, and yet it is certain beforehand to be 

 verified in the strict sense of the word. How, then, shall we know 

 when it has been extended as far as is legitimate? Simply when it 

 ceases to be useful to us t..,when we can no longer use it to pre- 

 dict correctly new phenomena. We shall be certain in such a case that 

 the relation affirmed is no longer real, for otherwise it would be fruit- 

 ful ; experiment without directly contradicting a new extension of the 

 principle will nevertheless have condemned it. 



Physics and Mechanism. Most theorists have a constant predilec- 

 tion for. explanations borrowed from physics, mechanics, or dynamics. 

 Some would be satisfied if they could account for all phenomena by 

 the motion of molecules attracting one another according to certain 

 laws. Others are more exact; they would suppress attractions acting 

 at a distance; their molecules would follow rectilinear paths, from 

 which they would only be deviated by impacts. Others again, such 

 as Hertz, suppress the forces as well, but suppose their molecules 

 subjected to geometrical connections analogous, for instance, to those 

 of articulated systems; thus, they wish to reduce dynamics to a kind 

 of kinematics. In a word, they all wish to bend nature into a certain 

 form, and unless they can do this they cannot be satisfied. Is Nature 

 flexible enough for this? 



We shall examine this question hereafter under the head of Max- 

 well's Theory. Every time that the principles of least action and 

 energy are satisfied, we shall see that not only is there always a 

 mechanical explanation possible, but that there is an unlimited num- 

 ber of such explanations. By means of a well-known theorem due to 

 Konigs, it may be shown that we can explain everything in an un- 

 limited number of ways, by connections after the manner of Hertz, 

 or, again, by central forces. No doubt it may be just as easily demon- 

 strated that everything may be explained by simple impacts. For 

 this, let us bear in mind that it is not enough to be content with the 

 ordinary matter of which we are aware by means of our senses, and 

 the movements of which we observe directly. We may conceive of 

 ordinary matter as either composed of atoms, whose internal move- 

 ments escape us, our senses being able to estimate only the displacement 

 of the whole; or we may imagine one of those subtle fluids, which 

 under the name of ether or other names, have from all time played 

 so important a role in physical theories. Often we go further, and 

 regard the ether as the only primitive, or even as the only true matter. 

 The more moderate consider ordinary matter to be condensed ether, 

 and there is nothing startling in this conception; but others only re- 



