How might definite spatial scale be determined on the basis of monocular optical flows? We review the extant literature on the perception of distances in depth from optical flow and find that head motion is essential for accurate performance. Accordingly, we propose a solution that entails the use of symmetries in the physically determined form of the trajectories of the head oscillating so as to generate optical flow. The innovation is the use of physically determined symmetries in trajectory forms (Bingham, in press). The hypothesized solution is also notable for the use of both time dimensioned optical variables and intrinsic units. Use of time dimensioned variables was suggested by the structure of the problem and is a strategy that has been widely used following the analyses of Lee (1974). Use of units intrinsic to human perception and action is simply in recognition of the ways that such distance information is used (Warren, 1984). This solution, if verified, would be the first entirely specifiable solution found for this ancient problem.