Athanasius’ letter to Amun (354) is a meditation on purity.  Defilement, he argues, occurs “when we commit sin, that foulest of things.”  That is what Jesus meant when He said that we are defiled by what comes out - out of the heart .

Bodily functions, by contrast, do not defile in the least.  Athanasius rivals Luther in his unabashed affirmation of the goodness of bodies and bodily secretions: “what sin or uncleanness,” he asks, is there “in any natural secretion - as though a man were minded to make a culpable matter out of the cleanings of the nose or the sputa from the mouth.  And we may add also the secretions of the belly, such as are a physical necessity of animal life.”

There can be no sin in natural bodily functions if “the Master who made the body willed and made these parts to have such passages.”  We are made by God’s hands, and “how could any defiled work proceed from a pure power?”