BPST INSTANTON AND SPIN FROM INEQUIVALENT QUANTIZATIONS

We present a simple alternative to Mackey's account of the (infinite) inequivalent quantizations possible on a coset space G/H. Our reformul ation is based on the reduction G-->G/H and employs a generalized form of Dirac's approach to the quantization of constrained systems. When applied to the four-sphere S-4 similar or equal to Spin(S)/Spin(4), th e inequivalent quantizations induce relativistic spin and a background BPST instanton; thus they might provide a natural account of both of these physical entities.