Chan translates as "stilling the thought." It also means "thought cultivation." "Thought" refers to investigating the meditation topic. "Stilling the thought" means, "At all times, wipe it clean, and let no dust alight." "Thought cultivation" is done by means of raising the meditation topic in thought after thought and never forgetting it. As it is said, "We do it in the morning, and we also do it in the evening." You should apply effort within your own nature and not seek outside. If you follow after any external state, it is easy to go astray. Only states that arise from your own nature are true states. I hope all of you will note this point clearly. Otherwise, you will be tricked by the demon king and end up as one of his followers.
This Dharma-door of "stilling the thought" requires that you apply continuous, unrelenting effort, without any interruption. In your meditation, you should as concentrated as a mother hen sitting on her eggs. You should also be as careful as a dragon guarding its pearl. Furthermore, you must be as patient as a cat waiting outside a mousehole. Chan meditation requires determination, sincerity, and perseverance. You cannot be arrogant and assume that you're higher and better than everyone else. If you have thoughts like these, that means a demon of insanity has possessed you, and your skill will never advance.
When you cultivate Chan, you should not indulge in idle thoughts. When you have idle thoughts, you don't gain any benefit, and you waste a tremendous amount of time. Sitting in Chan requires patience and perseverance. The secret of success in Chan meditation is patience; whatever you cannot endure, you must still endure. If you can endure to the ultimate point, then suddenly you can penetrate through and experience the clarity of enlightenment. If you lack patience and cannot endure bitterness and fatigue, then you will surrender to the states that you encounter. To give in like that is a great mistake; it is the great taboo in Chan meditation!