he word mishmar refers to the act of guarding someone closely, just as an officer or warden might keep watch over a prisoner. The phrase translated "with all diligence" (mikkol-mishmar) literally means "more than anything that might be guarded," and is used here to intensify the command to guard. Plainly put, this verse commands us to watch over the heart more than anything else.

Why? Because from the heart are totz'ot chaiyim - the "issues" of life. In the Tanakh, the word totz'ot is mainly used to refer to the borders of territories or the boundaries of a city. This verse is saying that from the heart of a person (lev) a "map" or "chart" to life is drawn. As the heart is either pure or corrupt, so will be the course of one's life.