Man has been formed of two different substances, the earthly and the heavenly. As his earthly frame is liable to ailments, so is the heavenly; and there are doctors for the treatment and cure of both. The doctors of the bodily ailments are the physicians, and those of the moral ailments are the Prophets and [later on] the Saints who are their successors. As a sick man would certainly die if not treated by a skilled physician, so a soul suffering from the moral diseases would certainly die, if not helped by a Prophet or a perfect Saint. As a physician examines the pulse to ascertain the disease of a patient, and recommends him to resort to one thing and abstain from another, with a view to restore physical equilibrium and health,—so also the Divine Messenger ascertains the moral ailments of the disciple, and prescribes different duties based on the Law according to his receptivity and capacity, recommending this, disallowing that, so as to reduce his inner perplexities and desires to a state of harmony required by the Law, and bring about moral health in the shortest possible time. As a sick man going against the instruc­tions of his physician gets worse and worse and has to die, so a moral patient disobeying the Law gets more and more perverse and has to perish through ignorance.—Letter 19.