52 



SCIENCE 



[N. S. Vol. XLVII. No. 1203 



superconductivity, Debye's theory of spe- 

 cific heats, the Eutherford nucleus atom, 

 the existence of chemical isotopes, Bohr's 

 theory, Moseley's law, Einstein's theory of 

 gravitation. I do not recall anything com- 

 parable with these in interest and impor- 

 tance which has appeared during the past 

 three years. "Whatever services science 

 may render to war, it is plain that a state 

 of war is not favorable to the progress of 

 science. Accordingly, the word "present" 

 in my title must be interpreted with some 

 latitude; it really applies to the state of 

 things before the peaceful labor of physi- 

 cists was interrupted by the duty of turn- 

 ing their attention to problems in applied 

 science whose solution is of immediate 

 urgency. 



No one can doubt that there has been 

 something very like a revolution in the 

 ideas and methods of theoretical physics 

 since the beginning of the twentieth cen- 

 tury. Much recent work of undoubted 

 significance would seem very strange to 

 Helmholtz and Lord Kelvin; and even in 

 some of our own contemporaries whose 

 tastes are conservative, it excites feelings 

 similar to those experienced by a Koyal 

 Academician before a cubist painting. On 

 the other hand, some of our younger and 

 more enthusiastic colleagues are inclined to 

 be impatient of what they call "classical" 

 theories (some of which were perfected in 

 the 1890 's), and to regard them as ex- 

 amples of superstition and logical punctilio 

 from which they have been happily freed. 



The truth is of course to be found be- 

 tween the two extreme views. We must 

 recognize that this is not the first change in 

 physical science which has seemed at the 

 time to be revolutionary. In the past, these 

 changes have never been so complete and 

 overwhelming as was expected by their 

 supporters, nor so abortive as hoped by 

 their opponents. In science, as in art, pol- 



itics and religion, the radicals are always 

 partly right and the conservatives never 

 wholly wrong; and the interplay and con- 

 flict between the two is of the very essence 

 of progress. 



One of the most striking things about 

 the modern beginnings of our science — the 

 preliminary formulation of the principles 

 of mechanics by Galileo, and their more 

 complete development by Newton — was 

 their almost immediate acceptance by all 

 who were not blinded by theological preju- 

 dices. This can not have been because they 

 were simple or easy to formulate, or the 

 world would not have had to wait so many 

 centuries for them. But the phenomena of 

 mechanics are directly and explicitly pre- 

 sented to us from our earliest childhood, 

 and have been so presented to our long line 

 of ancestors, human and pre-human. 

 Under given conditions, certain mechanical 

 actions are almost as confidently expected 

 (even by quite uninstructed persons) as if 

 their knowledge was of the a priori char- 

 acter that is attributed by many philoso- 

 phers to our mathematical and spatial con- 

 cepts. Even animals share this mechanical 

 knowledge. The instinctive movements of 

 a eat, which enable it to land upon its feet, 

 could scarcely be improved upon if it pos- 

 sessed a satisfactory knowledge of the con- 

 servation of angular momentum. The diffi- 

 culty of formulation was doubtless due to 

 the lack of recognition of the true character 

 of frietional and dissipative forces, and to 

 the obscuring of the idea of mass by the 

 more conspicuous property of weight. At 

 all events, when the principles are once 

 presented to the normal, intelligent, ob- 

 servant mind, they are quickly recognized, 

 and soon come to seem almost as axiomatic 

 as the attributes of space and number. 

 There can be little doubt of the reality of 

 this mechanical "intuition," be its origin 

 what it may. Whatever the philosophers 



