Unfortunately, if one mistakes the monkey mind for the mind of suchness (which is not the mind of birth and death), the vehicle of Zen Buddhism will not go very far. In fact it won’t move at all. Just what is the monkey mind? Here is what I was able to find.
“Just as a monkey swinging through the trees grabs one branch and lets it go only to seize another, so too, that which is called thought, mind or consciousness arises and disappears continually both day and night” (S. ii. 95).
We should recognize this monkey mind in ourselves and stop indulging in its monkey shines and deceptions. With another look at Zen, many of us imagine this monkey mind can be tamed by zazen. But this is only a temporary measure. It does nothing to help us transcend such a mind and realize absolute Mind which is the very essence or substance of our thoughts.
Modern Zennists, of course, are free to cherry-pick their Zen and often do bringing in terms such as “ordinary mind” or “no mind” as if to say, the Buddha really taught the monkey mind which is also to say, the mind of ignorance is the enlightened mind. This reflects a rather bad, anti-academic attitude amongst some Zennist since neither ordinary mind or no mind refer to the monkey mind. What is the ordinary mind?
“Mazu used the term pingchang-xin (J. heijôshin or byôjôshin) in its authentic meaning, i.e., the mind (or self) of no defilement and no discrimination, and he contrasted it with another term, shengsi-xin (J. shôji-shin, "the life-and-death mind (or self)), which to Hisamatsu is nothing other than defilement. Critics of Mazu's thought fail to see this point” (Shin'ichi Hisamatsu, Gishin Tokiwa, Christopher Ives, Critical Sermons of the Zen Tradition, 152 n. 59).
As for the subject of no mind.
“No mind means that there is no deluded, foolish mind; it does not mean there is no mind to discern false from true” (Thomas Cleary, The Original Face, p. 24).