THE MEANING OF PERCEPTION 



189 



intuitions " of space and time. Now it is useful to consider the 

 latter ideas for a moment, for this will enable us to see that the 

 categories have another aspect — they are mental constraints. 

 What, then, is abstract space ? Space, we may say for the 

 moment, has three dimensions: in whatever way a body may 

 move we can describe its position at any moment by placing it 

 in a " frame of reference." It is distant so much above or below 

 a plane (1) which stretches out horizontally to an indefinite 

 extent; it is also distant so much to the right or left from 

 another plane (2), which is at right angles to the first one, and is 

 also indefinite in extent; and, lastly, it is so much in front or 



Fig. 49. — The Three Co-ORDmATE_PLANES of Euclidian Space. 



behind a third plane (3), which is perpendicular to the first two. 

 We suppose, in making this three-dimensional frames of reference, 

 that we are situated at the point where all three planes intersect 

 each other. Plainly, then, if we move we carry the frame of 

 reference with us, and the position of the body we are observing 

 changes relatively to the three co-ordinates. 



Thus the motions of all bodies outside our own are relative to 

 our motions. Conceivably we might move in such a way as to 

 keep the frame of reference (which we carry with us) in the same 

 position relatively to a body which would be moving if we were 

 at rest, but which would appear to be at rest relatively to our 

 moving frame of reference. Motions of external objects, then, 



