Jamieson, Fausset and Brown on Matthew 15:17-18

17-18. Do not ye yet understand that whatsoever entereth in at the mouth, &c.--Familiar though these sayings have now become, what freedom from bondage to outward things do they proclaim, on the one hand; and on the other, how searching is the truth which they express--that nothing which enters from without can really defile us; and that only the evil that is in the heart, that is allowed to stir there, to rise up in thought and affection, and to flow forth in voluntary action, really defiles a man!