318 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



fact, exhibiting quantitative variations, but not reducible 

 to readings of speedometers, clocks, and meter sticks. For 

 science, however, motion is inseparable from the measure- 

 ment of motion, since it is defined as the quotient of space 

 divided by time, and these are in turn expressible only as 

 numbers. Hence, scientific motion takes on its meaning only 

 through a description of the activity of constructing, setting 

 up, and reading clocks, meter bars, and other instruments 

 of a more complicated sort. It is still empirical motion with 

 which the scientist is concerned, except that he is able to 

 measure unobservable motions, and to measure observable 

 motions more accurately. The fact that motion exhibits not 

 only features of magnitude, or intensity, but direction as 

 well, places it in the class of vector quantities, and permits it 

 to be handled according to the special laws determining the 

 addition, multiplication, and other operations of this kind 

 of quantity. The ordinary representation of a vector quan- 

 tity as a straight line with an arrow attached, exhibits 

 clearly the magnitude and directional features of motion. 

 Absolute character. The precise sense in which Newtonian 

 motion is absolute must be made clear. If, for the moment, 

 one calls the Newtonian theory the Absolute Theory, and 

 the empirical theory the Relational Theory, the differences 

 between the two points of view may be made clear by a quo- 

 tation from Rroad. "Absolute motion is the passing of a 

 body from one point of Absolute Space to another. Absolute 

 rest is the remaining of a body at a point of Absolute Space. 

 Relative motion has the same meaning on both theories; it 

 is just a change in the relative positions of two bodies. The 

 difference about it is that the Relationists say that all 

 motion simply is a change in the spatial relations of one 

 body to others, whilst the Absolutists say that there is 

 absolute as well as relative motion and that the two must be 

 distinguished from each other. On the Absolute Theory all 

 relative motion implies absolute motion, and is the appear- 

 ance of it to us, but a knowledge of relative motion does not 

 suffice to determine unambiguously the absolute motions 



