第3页 / 共212页

This primitive method of place specification deals only with

places on the surface of rigid bodies, and is dependent on the

existence of points on this surface which are distinguishable

from each other. But we can free ourselves from both of these

limitations without altering the nature of our specification of

position. If, for instance, a cloud is hovering over Trafalgar

Square, then we can determine its position relative to the

surface of the earth by erecting a pole perpendicularly on the

Square, so that it reaches the cloud. The length of the pole

measured with the standard measuring-rod, combined with the

specification of the position of the foot of the pole, supplies

us with a complete place specification. On the basis of this

illustration, we are able to see the manner in which a refinement

of the conception of position has been developed.