Kyozan asked Chuyu, "What does buddha-nature mean?"
Chuyu said, "I will explain it for you by allegory. Suppose there is a room with six windows. Inside there is a monkey. Outside, someone shouts, 'Monkey! monkey!' It immediately responds. If someone calls, 'Monkey!' through any of the windows, it responds just the same. It is just like that."
Kyozan said, "How about when the monkey is asleep?"
Chuyu descended from his Zen seat, grasped Kyozan and said, "O monkey, monkey, there you are!"
(Note how a Zen master takes a traditional Buddhist simile and turns it on its head. Bold, ruthless, and unexpected!
The traditional Buddhist metaphor is that the human "monkey mind" is the so-called "sixth sense." It's the mano-vijnana, the "discriminating consciousness" that takes up and processes the activity of the five physical senses -- sight, hearing, taste, smell, touch. The early Buddhists recognized it as its own distinct sense because of the way it creates reveries and dreams that are not directly based on the input of the five senses. Also, it is sometimes crazed and uncontrollable, like a monkey.
But here Chuyu is actually comparing the buddha-nature to a monkey that responds directly when called through any of the "six windows," including the mind. The Buddha nature itself described as a monkey! Strange, irreverent, and brilliant.
Thus, the monkey acts like a Zen Master as described by Takuan Soho. When called through any of the windows, he instantly replies without thinking.
However, Chuyu was wrong in one sense. In the widest regard, there are not just six windows, but billions. Why? Because there is really just one Buddha-monkey in all the billions of bodies endowed with minds and sense organs.)