THEORIES OF SCIENTIFIC CONCEPTS 155 



and time and the laws of gravitation apply to small bodies; 

 electrical laws and the principles for the ascertainment of 

 length through the application of meter sticks do not fit 

 when the phenomena are cosmical. But since progress in 

 physics consists in the gradual expansion of the known into 

 the realm of border-line events, we must expect that our 

 conceptual schemes will be subjected to shocks of varying 

 degrees of intensity, to the extent to which we have not 

 modified them in favor of the probable novelties which they 

 will be called upon to explain. 



The second fact to be recognized is the necessity for an 

 empirical attitude in physics. The physicist "recognizes 

 no a priori principles which determine or limit the possibil- 

 ities of new experience. Experience is determined only by 

 experience. This practically means that we must give up 

 the demand that all nature be embraced in any formula, 

 either simple or complicated." 1 And it also means that 

 concepts, which are the sole instrument for exploring ex- 

 perience, should themselves be empirically defined. "Con- 

 cepts can be defined only in the range of actual experiment, 

 and are undefined and meaningless in regions as yet un- 

 touched by experiment. It follows that strictly speaking 

 we cannot make statements at all about regions as yet un- 

 touched, and that when we do make such statements, as 

 we inevitably shall, we are making a conventionalized extra- 

 polation, of the looseness of which we must be fully con- 

 scious, and the justification of which is in the experiment 

 of the future." 2 



The safest procedure, in the face of such a situation, is 

 to adopt an operational theory for the definition of concepts. 

 Concepts must be defined in terms of the processes by 

 which the objects in question are found. "We may illus- 

 trate by considering the concept of length: what do we 

 mean by the length of an object? We evidently know what 

 we mean by length if we can tell what the length of any 

 and every object is, and for the physicist nothing more is 



1 Ibid., p. 3. 2 Ibid., p. 7. 



