Albert Barnes on Matthew 15:17

Do not ye yet understand, etc. The meaning of this may be thus expressed: The food which is eaten does not affect the mind, and therefore cannot pollute it. The doctrine of the Pharisees, that neglect of washing and of similar observances defiles a man, cannot be true. Those things pertain to the body as much as food does, and they cannot affect the soul. That must be purified by something else than external washing; and it is polluted by other things than a neglect of mere outward ceremonies. The seat of corruption is within. It is the heart itself; and if men would be made pure, this must be cleansed. If that is corrupt, the whole man is corrupt.

(l) "mouth goeth" Lk 6:45, Jas 3:6