Then he called the crowd to him and said to them, "Listen and understand:  it is not what goes into the mouth that defiles a person, but it is what comes out of the mouth that defiles."  Then the disciples approached and said to him, "Do you know that the Pharisees took offense when they heard what you said?"  He answered, "Every plant that my heavenly Father has not planted will be uprooted.  Let them alone; they are blind guides of the blind.  And if one blind person guides another, both will fall into a pit."  But Peter said to him, "Explain this parable to us."  Then he said, "Are you also still without understanding?  Do you not see that whatever goes into the mouth enters the stomach, and goes out into the sewer?  But what comes out of the mouth proceeds from the heart, and this is what defiles.  For out of the heart come evil intentions, murder, adultery, fornication, theft, false witness, slander.  These are what defile a person, but to eat with unwashed hands does not defile."

Jesus corrects our ideas about the origin of sin and immorality.  Sin has its origin in the heart.  We must reject the modern fallacy that if an evil intention is not actually performed (e.g., fornication) then no sin has been committed.  In fact if one even looks lustfully at a woman they have committed adultery with her in their heart.  (Matthew 5.28)  The battleground then is the human heart, it is there where we must engage our weakness and keep vigil.  But how do we keep our intentions from what is evil except by keeping them on the good?  The heart will produce thoughts and intentions, it is only a question of which kind.