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Master Daie (Ta-hui) says this about kufu: In training ourselves to solve koan, we should neither make guesses or comments nor try to understand them. It is unnecessary to know the meanings of the words or justify our attitudes toward the koan presented to us. On the contrary, we should neither be empty and tranquil nor expect to be enlightened. It is still worse to be absent-minded. Whether we walk, dwell, sit, or lie down, we should always be one with the koan and try to keep in touch with them all the time.17 In the Mumonkan, Master Mumon Ekai (Wu-men Hui-k’ai) states, Arouse your entire body with its three hundred and sixty bones and joints and its eighty-four thousand pores of the skin. Summon up a spirit of great doubt and concentrate on this word “mu.” In order to do so, hold to the problem from morning to night without letting it go even for one second, and become one with the word “mu” (void) with all your strength.
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Omori Sogen (Introduction to Zen Training: A Physical Approach to Meditation and Mind-Body Training (The Classic Rinzai Zen Manual))