Alert readers will have realized that up to now, I have only been discussing half of the dialog between Nangaku and Baso. After Nangaku asked, "How can you become a buddha by polishing a roof tile?," Baso asked, "What do you mean by that?"
Nangaku said, "Think about driving a cart. When it stops moving, do you whip the cart or the horse?"
Baso said nothing.
Nangaku said, "Do you want to practice zazen or sitting Buddha? If you understand zazen, you will know that Zen is not about sitting or lying down. If you want to learn sitting Buddha, know that sitting Buddha is without any fixed form. Do not use discrimination in the non-abiding dharma. If you practice sitting as Buddha, you must kill Buddha. If you are attached to the sitting form, you are not yet mastering the essential principal."
Baso heard this admonition and felt as if he had tasted sweet nectar.
I should also point out here that Nangaku was a student of Hui-Neng, the Sixth Patriarch. The lineage that descended from him through Baso has produced two of the five major schools of Zen, including the Rinzai tradition. But he was not admonishing against the practice of zazen. The question "Do you whip the cart or the horse?" has been used often by many Zen Masters, including Dogen. "Killing the Buddha" is not as blasphemous as it sounds at first, but instead killing our concept of a "thing" with fixed form and abode that we call "Buddha."
Zazen is the practice of the Buddha. Zazen is the ultimate practice. It is indeed the true self. That is why Dogen said, "When the polished tile became a mirror, Baso became Buddha. And when Baso became Buddha, Baso immediately became the real Baso. And when Baso became the real Baso, his sitting in meditation immediately became real zazen."