The Recorded Sayings of Chan Master Huangbo Xīshān

Huangbo Xiyun stated: In this Zen tradition of ours, transmitted from patriarch to patriarch since ancient times, we have never taught people to seek knowledge or understanding. We only speak of "learning the Way," though this phrase itself is merely a provisional means of guidance, for the Way fundamentally cannot be learned. If one harbors the idea of learning and understanding, it instead becomes an obstruction to the Way... Above all, do not engage in intellectual understanding.

He also said: The minds of the ancients were sharp; upon hearing just a single word, they immediately ceased all [conventional] learning. Hence, they were called 'idle wayfarers who have ceased learning and attained non-action'. People nowadays only desire to know more and understand more, broadly seeking textual meanings and interpretations, calling this 'practice'. They do not realize that this very knowing more and understanding more becomes an obstruction. The Essential Method of Transmitting the Mind (Huangbo Chuanxin Fayao)

Give you a single green leaf, and you dig out a nest of sweet potatoes – that is realization. Asking others for a huge pile of sweet potatoes while failing to see even a single leaf [yourself] – those are sweet potatoes dug up by others. There are also those who collect a whole house full of leaves, yet die without ever discovering a single sweet potato. They haven't even attained comprehension.