24 THE PHILOSOPHY OF BIOLOGY 



We imagine three lines at right angles to each other 

 to extend indefinitely out into space, and we think 

 of ourselves as being situated at the point of intersection 

 of these three straight lines. If anything moves in 

 the universe outside ourselves we can resolve this 

 motion into three components, each of which is to be 

 measured along one of the axes of our system of co- 

 ordinates. But any motion whatever in the universe 

 outside ourselves can be represented equally well by 

 supposing that the origin of the system of co-ordinates 

 has been changed ; that is, by supposing that we 

 have changed our position relative to the rest of the 

 universe. Therefore motion outside ourselves is not 

 to be distinguished from a contrary motion of our own 

 body — a statement of the "principle of relativity" — 

 except that any change outside ourselves may be 

 distinguished from that compensatory change in the 

 position of our body which appears to be the same 

 thing, by the absence of the intuition that we have 

 expended a certain quantity of energy in producing 

 the change. Conscious motion of our own body is 

 something absolute ; all other motion is relative. 



So far we have been speaking of our crude bodily 

 motion, but a very little consideration will show that 

 our knowledge of space attained by scientific measure- 

 ments depends just as much on our intuition of our 

 bodily activity, and its direction ; the measurement 

 of a stellar parallax, or that of the meridian altitude 

 of the sun, for instance, by astronomical instruments, 

 involves bodily exertion, though of a refined kind. 

 Three-dimensional space, that is our space, therefore 

 represents the manner of our activity, just as convex 

 two-dimensional space represents the manner of the 

 activity of the Infusorian, and one-dimensional space 

 would represent the manner of activity of an animal 



