John Gill Commentary Matthew 15:17

"Perceive ye not, that whatsoever goeth into the mouth passeth into the belly, and is cast out into the draught?" — Matthew 15:17 (ASV)

Do not you understand
You must understand, you cannot be so ignorant,

that whatsoever enters in at the mouth, goes into the belly, and
is cast out into the draught?
that is, that whatsoever food a man takes in at his mouth, he swallows down, and it is received into his stomach; which, having performed its office, the grosser parts go down into the belly, and passing through the bowels, are evacuated into the vault, or privy, "purging all meats", as Mark says; for that only receives the filth and excrementitious matter; so that what is left in the body is pure, wholesome, and nourishing:

nor can any part of what goes into a man defile him, because it only enters into the body, and passes through it; and, as Mark says, "entereth not into the heart", which is the seat of moral impurity; so that no moral pollution can be contracted by eating any sort of food, even though it should not be clean itself, nor be eaten with clean hands.