Why does spacetime have three spatial dimensions and one time dimension? The question has no answer in standard physics — dimensionality is simply observed. Pentagon Physics derives it. Three spatial dimensions are forced by the condition that the icosahedron (the 3D realisation of five-fold symmetry) is a Platonic solid only in three dimensions. The argument uses two independent paths: the ζ(3) argument (exchange space must be three-dimensional for the Apéry constant to appear in the α series) and the icosahedral uniqueness argument (the icosahedron achieves maximum symmetry exactly in three dimensions). Time is the direction of the self-referential attractor.