Know that the world of sitting practice is far different from other worlds.  Clarify this, then activate the way-seeking mind [...] Study the world at the very moment of sitting.  Is it vertical or horizontal?  At the very moment of sitting, what is sitting?  Is it an acrobat's graceful somersault or the rapid darting of a fish?  Is it thinking or not thinking? Is it doing or not doing? Is it sitting within sitting?  Is it sitting within body-mind?  Is it sitting letting go of sitting within sitting, or letting go of sitting within body-mind? Investigate this in every possible way [...] There is sitting with the mind, which is not the same as sitting with the body.  There is sitting with the body, which is not the same as sitting with the mind.  There is sitting letting go of body-mind, which is not the same as sitting letting go of body-mind.

-Dogen