"Consider the geometry of a three-dimensional homogeneous and isotropic space. ...[G]eometry is encoded in a metric g_{ij}(\mathbf{x}) (with i and j running over the three coordinate directions), or equivalently a line element ds^2 \equiv g_{ij} dx^i dx^j, with summation over repeated indices... ds is the proper distance between \mathbf{x} and \mathbf{x}+\mathbf{dx}, meaning... the distance measured by a surveyor who uses a... Cartesian [coordinate system] in a small neighborhood of... point \mathbf{x}.) One... homogeneous isotropic three-dimensional space with positive definite lengths is flat space, with line elementds^2=d\mathbf{x}^2...The coordinate transformations that leave this invariant are... ordinary three-dimensional rotations and translations. ...Another ...possibility is a four-dimensional with some radius a, with line elementds^2=d \mathbf{x}^2+dz^2,\;\;z^2 + \mathbf{x}^2 = a^2,...Here the transformations that leave the line element invariant are four-dimensional rotations; the direction of \mathbf{x} can be changed to any other direction by a four-dimensional rotation that does not change z. ...[T]he only other possibility (up to a coordinate transformation) is a hyperspherical surface in four-dimensional , with line elementds^2 = d\mathbf{x}^2 - dz^2,\;\;z^2 - \mathbf{x}^2 = a^2,...where a^2 is (so far) an arbitrary positive constant. The coordinate transformations that leave this invariant are four-dimensional pseudo-rotations, just like s, but with z instead of time."