430 SCIENCE PROGRESS. 



connection with the dominating mechanical conception of 

 nature. But a fundamental part of the new thought was 

 thereby lost. 



Half a century has been necessary to mature the con- 

 viction that this hypothetical addition to the law of energy 

 is in reality no deepening of insight but a renunciation of 

 its most important aspect : its freedom from every arbitrary 

 hypothesis. It was not, however, the recognition of this 

 general fact which actually brought about this advance, 

 namely, the rejection (as far as it has gone) of the me- 

 chanical explanation, but rather the final failure of all 

 attempts to interpret satisfactorily by the mechanical treat- 

 ment the phenomena connected with the remaining forms 

 of energy. 



But you will be impatient to learn how it is to be 

 possible by such an abstract idea as energy to form a 

 conception of the universe which shall be comparable, in 

 clearness and intuitiveness, with the mechanical one. The 

 answer ouo-ht not to be difficult. What do we know of the 

 physical world ? Obviously only what is vouchsafed us 

 through our organs of sense. But under what conditions 

 are these organs set in action ? Turn the matter as we 

 may, the only principle we find common to all is this : 

 The Organs of perception react in response to differences of 

 energy betzveen them and the surroundings. In a world in 

 which the temperature was everywhere that of our bodies 

 we should have no experience of heat, just as we have no 

 perception of the constant atmospherical pressure under 

 which we live. Not till we have produced a space of 

 different pressure do we become aware of its existence. 



Well and good, this you will be ready to admit. But 

 you will not be so ready to abandon matter, for energy 

 must of course have a carrier. But I ask in return : 

 Why ? Since our total knowledge of the outer world con- 

 sists in its energy relations, what right have we to assume in 

 this very outer world the existence of something of which 

 we have had no experience ? But energy, it has been urged, 

 is only something thought of, an abstraction, while matter 

 is a reality ; exactly the reverse, I reply. Matter is a thing 



