Charles Ellicott 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)

Is cast out into the draught — The word is used in its old English sense, equivalent to a drain or sewer (see 2 Kings 10:27). St. Mark adds the somewhat perplexing words, purging all meats (Mark 7:19), which is explained in the note on that verse.

The principle implied is that a purely physical process, from beginning to end, cannot in itself cause moral defilement. Of course, it was possible for the appetites connected with that process to bring the taint of moral evil. However, these appetites were already present before the food and are counted among the things that come out of the heart, not into it.