Mr. R. F. Muirhead on the Laws of Motion. 481 



experimental Law or Hypothesis of Dynamics (applied), 

 which enables us to give to time-measurement such a specifi- 

 cation that durations of time, as well as other dynamical 

 magnitudes, are made to depend ultimately for their measure- 

 ment solely on space-measurement and observations of coinci- 

 dences in time. 



These conclusions we have arrived at by assuming that only 

 kinetic specifications for the measurement of force, mass, and 

 time, and only a kinetic definition of ' " true rest " are admissible. 



Before attempting to justify these assumptions, it may be 

 expedient to devote a few paragraphs to a general considera- 

 tion of the idea of our method. A theory is an attempt to 

 dominate our experience ; it is a conception which may enable 

 us, with as little expenditure of thought as possible, to 

 remember the past and forecast the future. 



The theory of Universal Gravitation is an example of a 

 very successful attempt, perfectly successful so far as it has 

 been tested. So with the Euclidean Geometry. 



On the other hand, we have theories which have been found 

 useful to enable us to dominate one region of experience, 

 while they break down in certain directions. The Newtonian 

 Emission Theory of Light is an example. There are others 

 which, if they do not break down absolutely, involve the mind 

 in difficulties hitherto unsolved; e. g. the " elastic-solid " 

 Wave Theory of Light. 



Theories which are found to break down when applied to 

 their full extent, as well as theories which have not been 

 sufficiently tested, are often called " working hypotheses/'' 



The only merits or demerits a theory can have arise from 

 these two desiderata : (1) it must not be contradicted by any 

 part of our experience ; (2) it must be as simple as possible *. 



Thus, for example, consider the two rival theories : (1) 



that the earth has a certain amount of rotation about its 



axis ; (2) that it has no rotation. The latter will be found to 



agree perfectly with our experience, provided we assume as a 



new physical law that there is a repulsive force of magnitude 



co 2 r away from the Earth's axis at every point of space, and 



dr 

 also at every point a force 2co -r. at right angles to the axis 



of the Earth, and to the shortest distance of the point from 



* Since physical theories form an organic whole, of course these quali- 

 ties must be considered with reference to the body of physical theory as 

 a whole. Thus, of two theories, one may, taken by itself, be less simple 

 than another, and yet be preferred to it, because the whole body of 

 physical theory becomes simpler when it is adopted. Sometimes, too, a 

 theory may be preferred because it seems to promise better for the future. 



