"I have come to know that thoughts, like water, will stay on course if we make a place for them to go. Otherwise, our thoughts follow the course of least resistance, always seeking the lower levels. Probably the greatest challenge and the most difficult thing you will face in mortal life is to learn to control your thoughts. In the Bible it says, as a man ‘thinketh in his heart, so is he’ (Prov. 23:7). One who can control his thoughts has conquered himself. As you learn to control your thoughts, you can overcome habits, even degrading personal habits. You can gain courage, conquer fear, and have a happy life.
Boyd K. Packer
BYU, Speeches of the Year, 26 Sept. 1967