### „The value of the intrinsic approach is especially apparent in considering 3-dimensional congruence spaces... The intrinsic geometry of such a space of curvature K provides formulae for the surface area S and the volume V of a "small sphere" of radius r, whose leading terms are 3)S = 4 \pi r^2 (1 - \frac{Kr^2}{3} +...),

V = \frac{4}{3} \pi r^3 (1 - \frac{Kr^2}{5} +...).“

— Howard P. Robertson American mathematician and physicist 1903 - 1961