So the most difficult thing is always to keep your beginner's mind. There is no need to have a deep understanding of Zen. Even though you read much Zen literature, you must read each sentence with a fresh mind. You should not say, "I know what Zen is," or "I have attained enlightenment." This is also the real secret of the arts: always be a beginner. Be very very careful about this point. If you start to practice zazen, you will begin to appreciate your beginner's mind. It is the secret of Zen practice.
In Japan we have the phrase shoshin, which means "beginner's mind." The goal of practice is always to keep our beginner's mind. Suppose you recite the Prajna Paramita Sutra only once. It might be a very good recitation. But what would happen to you if you recited it twice, three times, four times, or more? You might easily lose your original attitude towards it. The same thing will happen in your other Zen practices. For a while you will keep your beginner's mind, but if you continue to practice one, two, three years or more, although you may improve some, you are liable to lose the limitless meaning of original mind.
For Zen students the most important thing is not to be dualistic. Our "original mind" includes everything within itself. It is always rich and sufficient within itself. You should not lose your self-sufficient state of mind. This does not mean a closed mind, but actually an empty mind and a ready mind. If your mind is empty, it is always ready for anything; it is open to everything. In the beginner's mind there are many possibilities; in the expert's mind there are few.
In the beginner's mind there is no thought, "I have attained something." All self-centered thoughts limit our vast mind. When we have no thought of achievement, no thought of self, we are true beginners. Then we can really learn something. The beginner's mind is the mind of compassion. When our mind is compassionate, it is boundless. Dogen-zenji, the founder of our school, always emphasized how important it is to resume our boundless original mind. Then we are always true to ourselves, in sympathy with all beings, and can actually practice.