Human hand movements obey the two-thirds power law: velocity scales with curvature raised to the negative one-third power. Geometrically, this means motor trajectories are geodesics of an equi-affine metric. But there is a technical obstruction: in standard differential geometry, this metric has no intrinsic definition on the plane because acceleration is not tensorial under general diffeomorphisms. Patrick Iglesias-Zemmour proposes a radical reframing. Replace the smooth plane with the \