The training of a bodhisattva is: when looking at some color, to ask what it is that is being seen; when hearing some sound, to ask what it is that is being heard. This is called "facing inward of the Buddhas."
When you get up in the morning, however much business there may be waiting, first affirm this one thought, first turn to this meditation on seeing and hearing. After that, engage in the activities of the day. When going to have a meal or a drink, first of all try to bring this one thought to the fore and make a meditation on it. When you go to wash your hands, try to bring this thought uppermost in your mind and meditate on it. When last thing at night you are going to lie down, sit for a bit on the bedclothes and try to bring this thought to the fore and meditate; then lie down to sleep. This is practicing the true path of Buddhas and bodhisattvas. Whip up your enthusiasm for it by realizing how if you fail to grasp your true nature as one with the nature of Buddha, you will be lost in the wheel of continual rebirth, circling endlessly in the Six Worlds.
From the beginning, you must learn to put your whole heart into this basic meditation, going ahead with each thought and practicing on each occasion as it comes up. Keep up the right line of meditation: when you walk, practice while walking; when you sit, practice while sitting; when talking to people, practice while talking. When there is not talking and things are quiet, then you can meditate more intensely. When you look at things, ask yourself what it is that you see; when you hear things, ask yourself what it is that you hear. When things get very rushed so that you easily get swept away by them, ask yourself what this is, that you should get swept away by it? And even if you do get swept away, don't give up your meditation. If you get ill, use the pain as the seed-subject for your meditation.
Be aware of this heart of yours. See that it does not weaken, and thus continue to go forward. In fact when things are quiet, it corresponds to the time when warriors are safe within the castle, when they must train themselves in tactics and strategy. They practice with courage and sincerity. You must meditate with just such a strong resolve. You may not have the power of Buddhas yet, but you are one of those who are on the Way of all the Buddhas.
It is a fact that little enlightenment obstructs great enlightenment. If you give up any little enlightenment you may have, and do not clutch it to yourself, then you are sure to get great enlightenment. If when you have a little enlightenment, you take that as a seed and go steadily forward, further and further with your practice, then the great profit of all the Buddhas becomes fully manifest. You will naturally pass through the barrier riddles set by the patriarchs. Now indeed, individual application and universal principle are in accord, action and understanding are not separate. You attain the state of great release, the great freedom. It is for this that stress is laid on maintaining the practice.
When you have penetrated into the truth wholly, all the powers of the Way are brought to fulfillment, all beings everywhere are blessed whenever any opening presents itself. There may be mistakes and lapses; legs are weak and the path slippery. If you don't get up when you've fallen down, surely you'll be destroyed. If through falling, you pull yourself up, and falling again, pull yourself up again, finally you do reach the goal.
Intensifying the meditation practice in the way described, when the practice becomes clear and mature, you finally return to your fundamental nature. This is what is called Becoming Buddha. When it is said in Zen, See your own Nature to Be Buddha, this is what is meant. At the beginning, owing to the one delusion, the True Nature which should face inward, is made to circle outwardly in the Six Paths: of hell, hungry ghosts, animals, demons, people, and heavens. This is not just a question of reincarnation. In one single day here people are rising and sinking in it. When the heart is right and avoids wrong, that is existing as a person. When others oppose one and hatred arises, that is the demon realm. When one has sticking attachment for what one likes, he becomes a hungry ghost. When the heart gets stuck in fixating on material things, one is in the animal realm. If attachment is strong, if the flames of anger do not cease, then this is hell. All this is losing the path of humanity.
When the heart is peaceful and not thinking of material things, there is inner purity. Now, though in a human body, the heart is said to be sporting in heaven. But in general, people do not realize how they are circling about in these states during a single day.
If in the course of the day there arises some resolve to practice, then in that heart, seeds of anger, greed, and folly will be destroyed. One who strives to intensify practice finally attains realization; even before he does so, since the heart has calmed, he passes beyond temporary joys of human and heavenly worlds, and ascends to a higher state.
The Zen realization of Seeing the Nature is the very crown of all Buddhahood. One who has their heart set on this is already a baby Buddha. This is called the highest, noblest, and very first dharma. Step by step it must be faithfully followed out.