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.

GLOSS.a It says therefore into his heart, that is, into his mind, which is the principal part of his soul, on which his whole life depends; wherefore it is necessary, that according to the state of his heart a man should be called clean or unclean, and thus whatsoever does not reach the soul, cannot bring pollution to the man. Meats therefore, since they do not reach the soul, cannot in their own nature defile a man; but an inordinate use of meats, which proceeds from a want of order in the mind, makes men unclean. But that meats cannot reach the mind, He shows by that which He adds, saying, But into the belly, and goeth out into the draught, purging all meats. This however He says, without referring to what remains from the food in the body, for that which is necessary for the nourishment and growth of the body remains. But that which is superfluous goes out, and thus as it were purges the nourishment, which remains.

The Lord begins by chiding them, wherefore there follows, Are ye so without understanding also? Then the Lord shows them what was hidden, saying, Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot make him common?