صفحة 173 من 212

Perhaps the reader will wonder why we have placed our "beings" on

a sphere rather than on another closed surface. But this choice

has its justification in the fact that, of all closed surfaces,

the sphere is unique in possessing the property that all points

on it are equivalent. I admit that the ratio of the circumference

c of a circle to its radius r depends on r, but for a given value

of r it is the same for all points of the "worldsphere"; in other

words, the "world-sphere" is a "surface of constant curvature."