On this account, therefore, in the law concerning leprosy the great and wise Moses speaks of motion and its further progress and diffusion as unclean, but of tranquillity as pure. For he says, "If it be diffused over the skin the priest shall pronounce him polluted. But if the bright colour remain in its place and be not diffused, he shall pronounce him clean." Leviticus xiii. 12. So that, as tranquillity is an abiding of evils and of the passions within the soul (for that is what is intimated by leprosy), it is not liable to reproach; but its motion and progress are of necessity open to accusation.