Our wicked thoughts and affections, words and actions, defile us, and these only. As a corrupt fountain sends forth corrupt streams, so does a corrupt heart send forth corrupt reasonings, corrupt appetites and passions, and all the wicked words and actions that come from them.
A spiritual understanding of the law of God, and a sense of the evil of sin, will cause a man to seek for the grace of the Holy Spirit, to keep down the evil thoughts and affections that work within.