4 9 6 D O C . 32 9 F U N D A M E N T A L C O N C E P T S OF P H Y S I C S bird's-eye view of the development of the fundam ental concepts of physics up to the present time. Science strives to understand the connection between the facts observed by the senses, that is. to set up logical constructions of concepts In which such connections shall appear as logi- cal consequences. There Is liberty as far as the choice of concepts and rules of construction are concerned. This choice Is Justified solely by the re- sults. that Is. it must account cor- rectly for the relations. At the outset, physics took over the concepts of number, space, tim e. body, from pre-scientific thought and en- deavored to gel along with these Ideas. First, there arose the doctrine of the space relations of bodies without ref- erence to time changes: Euclidean geometry To have created this first logical system of concepts, which con- cerns Itself with natural objects. Is the Immortal merit of the Greeks. It was followed by the doctrine of space change of bodies with tim e this was the classical mechanics founded by Galileo and Newton, which had for Itsfoundation the geometry of Euclid. This doctrine was established In ________________ _ the first Instance to explain the motions of the heavenly bodies. Its foundations are: a material point moves u n i f o r m l y In a straight line as long as It Is at a great enough distance from all others. If other bodies are sufficiently near, the point moves with an acceleration wholly determined by Its location relative to the other masses. The determination of the degrees of this accel- eration from the poel- tion of the o t h e r bodies remained sub- ject to special hy- potheses c o n e ernlng the nature of the mu- tually Interacting forcee. One of these hypotheses was that of gravitation whose complete mathematical expres- sion was given by Newton . This strictly causal scheme was des- tined to give more than an explanation of mechanical phenomena In their re- stricted sense. Other changes . too. In bodies not directly recognisable as me- chanical. could In thought be Inter- preted as movement and equilibrium among the more minute building stones, such as changes In aggregate conditions, tem perature. chemical change. This endeavor to reduce all processes to mechanics led necessarily to the atomic theory. By the exten- sion of hypotheses about forces It seemed possible to consider all phe- nomena as strictly causal and refer- able to mechanics. This program, already divined by the great m aterialists of Greek an- tiquity. reads thus: reality consists ex- clusively of point-masses subject to no change other than motion which proceeds according to the rules of Newton. Marvelous results were reached on this basis. Celestial mechanics, tech- nical mechanics, the theory of heat, the theory of crystals, and, further- more, chemistry, flourished on this foundation without encountering any special difficulties In principle. Even the theories of electro-magnetism and light appeared at first to fit In with- out any Inconsistency. Today the ex- istence of unchangeable elementary' bodies (electrons, protrons appears an assured fact. And yet today we have definite knowledge that Newton's fundamental concepts and hypotheses only approx- imate the truth. At first it was the laws of electricity and light which necessitated the coining of concepts fundamentally new. When In the first half of the nineteenth century the un- riulatory character of light became evi- dent. this could still be accepted as the motions of an hypothetical body, the luminiferous ether. But the more accurately the properties of light be- came known, the more difficult It proved to ascribe to the luminifer- ous ether mechanical properties ex- plaining the facts and not self-contra- dictory. According to the assumption made, the ether was a kind of mate- rial substance which had little resem- blance to the "tangible and ponder- able" matter of the rest of physics. Newton's edifice thus lost Its original structural unity. When Faraday's and Maxwell's re- searches revealed the essential connec- tion between electricity and light. It soon became clear that a single funda- mental concept could not withstand th e pressure of facts, and this concept was that of forces acting directly at a distance and without requiring time. A new fundamental concept took Its [5] [4]