Matthew 23:26 - Ellicott's Commentary On The Whole Bible

That the outside of them may be clean also. — The implied premise is that “uncleanness” in its ethical sense was altogether distinct from the outward uncleanness with which the Pharisees identified it. If the contents of the cup were pure in their source and in their use, they made the outside “clean,” irrespective of any process of surface purification.