CONCEPTIONS AND METHODS OF PSYCHOLOGY 601 



for the evolution of primitive man, measurement is for the advance 

 of modern science. As a word selects similarities and ignores differ- 

 ences, so a measurement selects certain similarities from the con- 

 crete manifoldness of things. That such a great part of the world 

 can be described in terms of a few units of measurement, and that 

 this description should lead to such useful applications, is truly 

 marvelous and admirable. As I am writing these paragraphs, I have 

 received a manuscript in which the author explains that the fact 

 that the earth rotates on its axis in twenty-four hours, not varying 

 a second from day to day, is a conclusive proof that it was created 

 and set rotating by a benevolent being. If the days were shorter, 

 he says, we could not get our work done, and if the days were longer, 

 we should be too tired by night. It almost seems as though the 

 world were made in such comparatively rational fashion in order 

 that we may measure it. 



The physicist counts, and he measures time, space, and energy. 

 He has intractable matter with its seven and seventy elements, and 

 he may come across a substance as complex and perplexing as radium. 

 But by and large he can describe his world in certain quantitative 

 formulas. It is true that he accomplishes this in part by unloading 

 on psychology qualitative differences, such as colors and tones. So 

 much the more satisfaction to us if we can reduce them to quanti- 

 tative order. Perhaps we shall have only partial success; but it may 

 fairly be urged that psychology has done as much in this direction 

 in fifty years as physics accomplished until the time of Galileo, 

 or chemistry until the time of Lavoisier. 



The psychologist counts, and he measures time, space, and in- 

 tensity. Even if it were true I think it is not true that mental 

 magnitudes are not measurable, it would none the less be the case 

 that mental processes are described in quantitative terms. This is 

 attempted and accomplished in most of the researches published 

 in our psychological journals. They describe measurements and the 

 correlation of quantities; they show that a mental mechanics is 

 more than a possibility. 



The physical sciences have been primarily quantitative, and the 

 biological sciences are primarily genetic, but the physical sciences 

 must become genetic, and the biological sciences must become 

 quantitative. Psychology is from the start both quantitative and 

 genetic. It may indeed be claimed that it is the science in which 

 the genetic method has the most complete application. Every 

 mental state, and every form of activity, is the result of develop- 

 ment from previous conditions. If explanation, as distinguished 

 from description, is possible anywhere in science, it is possible 

 here. It is certainly difficult to penetrate by analogy into the 

 consciousness of the lower animals, of savages, and of children, 



