This mind is subject to change, for the good and the bad. Unfortunately this mind naturally delights bad more than good. And that is the origin of all problems – the thirst for evil. Hence, the mind that was once clean becomes unclean.
It then, without consent fills itself with evil intentions, thoughts, views and beliefs. As the mind goes unclean, the body commits unwholesome actions and the mouth unleashes grotesque speech. It is essential to train the mind because it determines the other two doors; action and speech. The mind is a circuit – you need to fix it first before you discipline your actions and speech.