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If everything has really gone smoothly, then I say that the

points of the marble slab constitute a Euclidean continuum with

respect to the little rod, which has been used as a "distance"

(line-interval). By choosing one corner of a square as "origin" I

can characterise every other corner of a square with reference to

this origin by means of two numbers. I only need state how many

rods I must pass over when, starting from the origin, I proceed

towards the "right" and then "upwards," in order to arrive at the

corner of the square under consideration. These two numbers are

then the "Cartesian co-ordinates" of this corner with reference

to the "Cartesian co-ordinate system" which is determined by the

arrangement of little rods.