Metropolis Death and Being, 33rd Day of Autumn, 525 M.E. (Helios):  To better understand all that was being implied by the talk of "polishing bricks" and "polishing mirrors" in the conversation between Nanyue and Mazu, it helps to know the tale of the Platform Sutra of the Sixth Patriarch. There is no doubt Nanyue and Mazu were intimately familiar with this story, and the mere mention of "polishing mirrors" was shorthand for the whole story. 

In the middle of one night, Ācārya Jinshū, the most intelligent and accomplished member of his monastery, took a lantern and posted the following poem on a corridor wall to express his understanding: 

The body is the bodhi tree,

The mind is like the stand of a clear mirror.

At every moment we work to wipe and polish it

To keep it free of dust and dirt. 

Master Enō, who had never been taught to read or write and had found work at the monastery as a kitchen attendant, had a boy from the temple read the poem to him.  Enō thought that comparing practice to keeping a mirror clean was too intellectual and artificial, so he recited his own poem to the  boy and had him secretly write it on the same wall for him. His poem read, 

In the state of bodhi there is no tree,

Nor does the clear mirror need a stand.

Originally we do not have a single thing,

Where could dust and dirt exist?

 All the monks were astonished at the excellence of Enō’s poem. 

"In general," Dogen wrote, centuries later, "we polish a mirror to make it into a mirror; we polish a brick to make it into a mirror; we polish a brick to make it into a brick; and we polish a mirror to make it into a brick." 

Dogen often spoke this way - in groups of four, seemingly contradictory statements. It often sounds confusing and deliberately obtuse at first, but on closer examination we can see that he's just looking at the same situation from four different philosophical viewpoints. Sometimes our idealism is like polishing a mirror in order to try and make some ideal mirror. Sometimes our behavior is as meaningless as trying to polish a brick into a mirror. Sometimes we realize some concrete thing through our practice, and sometimes our action transcends idealism completely.

Real examples of this occur in daily life. "There are also times," Dogen continued, "when we polish without making anything" (I can relate to that); "and there are times when it would be possible to make something, but we are unable to polish." 

I walked a full van Buren today, as pleasant and nice a late-summer day as we've had all year. The tropical storm that was crossing the Atlantic disappeared and died before it could make it to the Caribbean and cause trouble, so that's good. Tomorrow, it's back to polishing mirrors and warming cushions. Today, I beat the horse; tomorrow I beat the cart (or is it the other way around?).