We can talk of our practice and life in terms of form and emptiness, or delusion and enlightenment. We can also talk of both in terms of ground and space, earth and sky, heaven and earth.
In Inmo, Dogen comments on the phrase:
Those that fall to the ground get up relying on the ground. To get up without relying on the ground is, in the end, impossible.
One of the most difficult things for people to experience when they start zazen is their ungroundedness. They are caught up in a storm of thought and emotion. And through practice, they learn to experience falling back into the feeling, experiencing body, the ground.
We can experience this physically. If we sit properly, allowing our weight to drop down through our sit bones, then we can receive a reciprocal push upwards from the earth, up our spine and up through the top of our head.
This falling down and getting up is the activity of our lives. And in this activity, we actualise heaven and earth.