from the total Model only what is intelligible to a layman and only what makes some appeal to imagination and emotion. Thus our own backcloth contains plenty of Freud and little of Einstein. The medieval backcloth contains the order and influences of the planets, but not much about epicycles and eccentrics. Nor does the backcloth always respond very quickly to great changes in the scientific and philosophical level. Furthermore, and apart from actual omissions in the backcloth version of the Model,