Dogen concludes Genjo Koan with a story:

Zen Master Hōtetsu of Mayokuzan is using a fan. A monk comes by and asks, “The nature of air is to be ever present, and there is no place that air cannot reach. Why then do you use a fan?” 

The master says, “You have only understood that the nature of air is to be ever present, but you do not yet know the truth that there is no place air cannot reach.” 

The monk says, “What is the truth of there being no place air cannot reach?” 

At this, the master just carries on using the fan.  The monk bows in recognition of the master's wisdom.

Regarding this story, Dogen notes, "The real experience of potential and the vigorous road to the real experience is like this.  Someone who says that because the air is ever present we need not use a fan, or that even when we do not use a fan we can still feel the air, does not know ever-presence and does not know the nature of air.  Because the nature of air is to be ever present, the actions of those who know have made the earth manifest itself as gold and have ripened the Milky Way into curds and whey."