6 PREFACE. 



fashions science out of facts and is the indispensable 

 precondition of every important scientific advance. 



Heinrich Hertz,^ the discoverer of electric undula- 

 iions, had the same thought in mind when he said: 

 "We form inward representations or constructs of 

 outward objects, so constituted that the results that 

 follow logically and necessarily from the constructs 

 are in turn always constructs of the results flowing 

 naturally and necessarily from the objects." "These 

 constructs or mental images copied after familiar ob- 

 jects possessed of familiar properties, so constituted 

 that from their manipulation effects result similar to 

 those which we observe in the objects to be explained. 

 Experience teaches us that the requirements here 

 made can be fulfilled and that consequently such 'cor- 

 respondences' between reality and the supposed images 

 [or, as Hertz says, between nature and mind] actually 

 exist. Having succeeded in extracting from the ac- 

 cumulated experience of the past, representative 

 images or constructs fulfilling all these necessary re- 

 quirements, we can then reproduce by them in a short 

 space of time, as we might by models, results that 

 in the outward world require a long space of time for 

 their actualisation or can be produced only through 

 our personal intervention," etc. 



gravitation, of whose character Newton could form no con- 

 ception and hence was unwillinn to construct hypotheses 

 concerning it. Indeed, such a wholesale repudiation of hy- 

 potheses is antecedently incredible on the part of the inventor 

 of the emission-theory of light, in which, to speak of only 

 one daring conjecture, "fits" were ascribed to the luminous 

 particles. Compare Newton, Philosophiae Naturalis Prin- 

 cipia Mathematica, second edition, 1714, page 484. 

 1 H. Hertz, Die Principien der Mechanik. 



