Mark 7:19

(Lib. oct. Quaes. 73) For some things are joined to others in such a way as both to change and be changed, just as food, losing its former appearance, is both itself turned into our body, and we too are changed, and our strength is refreshed by it.b Further, a most subtle liquid, after the food has been prepared and digested in our veins, and other arteries, by some hidden channels, called from a Greek word, pores, passes through us, and goes into the draught.

When we read in Leviticus and Deuteronomy of the laws about food as clean and unclean (for the transgression of which we are censured by the legalists and by the Ebionites, who differ from them very little), we are not to think that the scope of the Scripture is found in any superficial understanding of them. For "whatever goes into a person from the outside cannot defile him, since it enters not his heart but his stomach, and so passes on." According to Mark, the Savior "declared all food clean," so we are not defiled when we eat those things declared to be unclean by those who still desire to be in bondage to the letter of the law. But we are then defiled when our lips, which ought to be bound with good judgment as we search for correct balance and weight, speak recklessly and discuss matters we ought not.

Do you not understand that everything entering from outside into a man cannot defile him because it does not enter his heart but his stomach, and is expelled into the latrine, purging all foods? All the passages of the Gospels among heretics and the perverse are full of scandals. And from this little statement, some accuse that the Lord, ignorant of physical disputation, thinks that all foods go into the stomach and are digested in the latrine, whereas the infused foods are immediately spread through the limbs, veins, and marrow of the nerves. Hence, we also see many who, due to a stomach defect, continually vomit, immediately expelling what they ingested after meals, and yet are corpulent because the more liquid food and drink spread through the limbs at first touch. But such men, in their desire to criticize another's lack of skill, show their own. Although the thinnest humor and liquid food, when it has been concocted and digested in the veins and limbs, descends to the lower parts through hidden pathways of the body, which the Greeks call πόρους, and goes into the latrine. He was saying, however, that what comes out of a man defiles the man. For from within, from the heart of men, evil thoughts proceed, adulteries, fornications, murders, thefts, covetousness, and others. "From the heart," he says, "come evil thoughts." Therefore, the principal part of the soul, not according to Plato in the brain, but according to Christ in the heart, is. And those are to be refuted from this assertion who think that thoughts are instilled by the devil and not born from one's own will. The devil can be an assistant and inciter of evil thoughts, but he cannot be an author. If, however, he, always lying in wait, inflames the light spark of our thoughts with his kindlings, we should not believe that he also scrutinizes the secrets of the heart, but judges from the body's condition and gestures what we are pondering inwardly. For example, if he sees us looking frequently at a beautiful woman, he understands that the heart is wounded by the dart of love.