SCIENCE, COMMON SENSE, AND DECENCY 3 



trie fields could be understood in terms of the familiar elastic properties. 

 At the present time relatively few students are well trained in the theories 

 of elasticity. The situation is thus reversed and to-day we explain the 

 properties of elastic solids in terms of the electrical forces acting between 

 their atoms. 



Every student of geometry constructs a mental model when he thinks 

 of a triangle. The mathematical lines that bound the triangle are supposed 

 to have no thickness. In other words, they are stripped of any properties 

 except those that we wish particularly to consider. 



Most of the laws of physics are stated in mathematical terms, but a 

 mathematical equation itself is a kind of model. We establish or assume 

 some correspondence between things that we measure and the symbols that 

 are used in an equation, and then, after solving the equation so as to 

 obtain a new relation, we see if we can establish a similar correspondence 

 between the new relation and the experimental data obtained from the 

 experiment. If we succeed in this we have demonstrated the power of the 

 mathematical equation to predict events. 



The essential characteristic of a model is that it shall resemble in certain 

 desired features the situation that we are considering. On this basis we 

 should recognize that practically any theory has many arbitrary features 

 and has limitations and restrictions imposed by the simplifications that we 

 have made in the development of the theory or the construction of our 

 model. 



Beginning with Einstein's relativity theory and Planck's quantum 

 theory a revolution in physical thought has swept through science. Perhaps 

 the most important aspect of this is that the scientist has ceased to believe 

 that words or concepts can have any absolute meaning. He is not often 

 concerned with questions of existence ; he does not know what is the mean- 

 ing of the question, "Does an atom really exist?" The definition of "atom" 

 is only partly given in the dictionary. Its real meaning lies in the sum total 

 of knowledge on this subject among scientists who have specialized in this 

 field. No one has been authorized to make an exact definition. Further- 

 more, we can not be sure just what we mean even by the word "exist." 

 Such questions are largely metaphysical and in general do not interest the 

 modern scientist. Bridgman has pointed out that all concepts in science 

 have value only in so far as they can be described in terms of operations 

 or specifications. Thus it doesn't mean much to talk about length or time 

 unless we agree upon the methods by which we are to measure length 

 and time. 



For many years, up to about 1930, the new physics based on the quan- 

 tum theory seemed to be fundamentally irreconcilable with the classical 

 physics of the previous century. Through the more recent development of 



