The incident concerning the polishing of a roof tile has conventionally been understood as a warning against sitting meditation. But I think this conventional understanding is not correct. After all, Nangaku did not tell Baso to stop his zazen. Instead, he was showing Baso the futility of trying to become that which he already possessed. When Zen Master Eihei Dogen said, "Polishing a tile to make a mirror is a meaningless use of effort" (Dharma Hall Discourse 281 in Eihei Koroku, Volume 4), he was not criticizing the practice (the polishing of the tile), he was talking about the meaninglessness of intending an outcome (making a mirror).
As previously pointed out, sitting with any goal or intention is not "just sitting" (shikan-taza), it is sitting with a goal or an intention. One doesn't "become" a Buddha, as everyone (and everything) already has buddha-nature, which is expressed by the very practice of shikan-taza. A mirror doesn't "become" a mirror, but a mirror's reflective property is made manifest by the practice of polishing.
So what is this practice of shikan-taza like, wherein one's buddha-nature can be made manifest? A monk once asked this of Zen Master Yaoshan, asking, "What are you thinking while in steadfast, immovable sitting?"
Yaoshan said, "I think of not thinking."
The monk said, "How do you think of not thinking?"
Yaoshan said, "Beyond thinking."
Commenting on this exchange, Shohaku Okumura noted that Yaoshan's first statement, "I think of not thinking," might be interpreted as "I think of the activity, or realm, of not thinking," while his second statement, "Beyond thinking," implies an aware mind that may include either thinking or not thinking, but is not attached to either. "In Dogen's view of zazen," Okumura writes, "one does not follow thoughts, nor stop them, but thoughts come and go freely. It cannot be called thinking, because thoughts are not grasped. It cannot be called not thinking because thoughts continue to come and go." The brain, an organ whose function is to squirt out thoughts, continues to function while one is in zazen. The one sitting just doesn't engage those thoughts that do arise.
During a dharma hall discourse, Dogen once said,
"Polishing a tile to make a mirror is diligent effort. How could the way of thinking within steadfast, immovable sitting be careless? If you want to visit that realm of glimpsing the ground of buddhahood, you should further come here and close your mouth in silence."Tell me great assembly, are Eihei (meaning Dogen himself) and the ancient ones the same or different? Try to say something and I'll see how you do. If you do not speak, I will speak for all of you."After a pause, Dogen struck his abbot's chair with the handle of his whisk, and got down from his seat (Dharma Hall Discourse 270 in Eihei Koroku, Volume 4).