2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/91336As is well known, classical systems approximate quantum ones -- but how well? We introduce a definition of a "distance" on classical and quantum phase spaces that offers a measure of their separation. Such a distance scale provides a means to measure the quality of approximate solutions to various problems. A few simple applications are discussed.LaTeX, 10 pages, no figuresQuantum Physicstext