As always, it is important to look at the context of this story. In Matthew’s gospel, the story follows discussions between Jesus and the Pharisees about the Jewish purity laws, and what makes a person clean or unclean. In the preceding passages, we have the Pharisees challenging Jesus about not keeping the law, (Your disciples don’t wash their hands before they eat!) but Jesus responds by challenging the Pharisees to think more carefully about what makes a person clean or unclean. He argues that it is not so much washing hands that makes a person clean and what goes into a person’s mouth, but rather what are the words that flow from one’s mouth and what are the thoughts that are in our hearts. This is what true purity is, he argues.