A long time ago, Zen Master Baso of Kiangsi Province trained under Master Nangaku. Just before the incident concerning the polishing of a roof tile, Nangaku intimately transmitted the mind-seal to Baso.
The incident concerning the polishing of a roof tile occurred as follows: While staying at Chuan-fa Temple, Baso constantly sat in zazen. This went on, day in and day out, in Baso's thatched hut for some ten years or more. We can only imagine what the humble thatched hut must have been like on a rainy night, and it is said that he never abandoned its freezing floor even when it was enveloped in snow.
One day when Nangaku came to Baso’s hut, Baso stood up to receive him. Nangaku asked him, "What have you been doing recently?"
Baso replied, "I have been just sitting."
Nangaku asked, "And what is the aim of your just sitting?"
Baso replied, "The aim of just sitting is to become a Buddha."
Nangaku promptly took a roof tile and began rubbing it on a rock near Baso’s hut.
Baso, upon seeing this, asked him, "Master, what are you doing?"
Nangaku replied, "I am polishing a roof tile."
Baso then asked, "What are you going to make by polishing a roof tile?"
Nangaku replied, "I am polishing it to make a mirror."
Baso said, "How can you possibly make a mirror by rubbing a tile?"
Nangaku replied, "How can just sitting possibly make you into a Buddha?"
First, we must realize that the mirrors of that time were made from metal, usually copper, that was cast in a flat, circular mold and then highly polished on one side. It was not uncommon for such mirrors, in time, to be broken up and recast into Buddhist statuary, which was left unpolished.
Second, Baso's statement, "The aim of just sitting is to become a Buddha," was deeply contradictory. The particular practice of meditation he specifically referred to was chih-kuan ta-tsuo (shikan-taza in Japanese), a Chinese colloquial phrase that literally translates as "just control yourself and sit there." Shikan-taza is usually translated as "just sitting!" or better, "just! sitting." This implies sitting in meditation without deliberately thinking of anything, or holding on to anything that naturally arises, or pushing away anything that naturally arises, and without trying to suppress any thoughts from arising. However, in reply to Nangaku’s question, Baso indicates that, in fact, he was deliberately holding something in his mind, namely, the goal of realizing Buddhahood, literally "to become a Buddha."
Finally, though, we come to the real paradox of this koan. It is clearly impossible for a roof tile to become a mirror, despite any effort to polish it. Was Nangaku implying that it was impossible for ordinary beings to become Buddhas? If the practicing of polishing is analogous to the practice of zazen, what is the point then of practice, if ordinary people cannot become Buddhas?
The first solution that arises in the mind to this question involves the contradiction in Baso's reply. If, in fact, shikan-taza involves no striving or aim, his sitting in zazen with the aim of becoming a Buddha was not shikan-taza. It was a useless practice, as useless as polishing a tile. By polishing a tile, Nangaku was earnestly endeavoring to encourage Baso in his practice by directly demonstrating the uselessness of maintaining an aim, any aim, in practice.
For hundreds of years now, this is how the story has been understood and transmitted. But soon, another question arises in the mind, namely, even in a practice without an aim, even with true shikan-taza, how can an ordinary person become a Buddha? How can a roof tile become a mirror? The rug is pulled out from under our feet again, and we find ourselves falling back into confusion.