Deep Commentary
For a practitioner who is still an ordinary person (having not yet attained Arahantship), no one can entirely trust their own mind. Why? Because it is a deluded mind, constantly changing and restless. It grasps at external objects all day long, jumping like a monkey from tree to tree or running wild like a horse in a field. Yogacara scholars call this "the monkey mind and horse will." Zen masters compare it to a wild ox, very difficult to tame. To tame it, one must first recognize its face. How can you tame what you don't know? Zen calls this "herding the ox." To herd it, you must know what the ox is and what it looks like. The ox represents wandering thoughts. Because they are wandering, the mind is always agitated and uneasy, changing unpredictably, making it hard to control. Hence, in verse 33, the Buddha says the mind of an ordinary person is "fickle and unsteady, so difficult to guard and control." In contrast, the wise—those who have achieved a level of awakening—subdue their minds and make them straight easily, "like a fletcher straightens an arrow." Because they are awakened, their gross wandering thoughts are gone, and they easily master their minds. With no wandering thoughts to agitate them, the sun of their mindfulness shines constantly, keeping their minds straight and true. Only then can they truly trust their own minds.
Zen Assistant
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