The classics of T’ai-Chi offer the following advice:
We should keep our thoughts on the spirit and not on the ch’i. When they are on the ch’i, then it is blocked. If there is ch’i, then there is no strength; without ch’i then essential hardness is achieved. (1)
The English word “spirit” comes from the Latin “spiritus”, meaning breath. To me, the advice from the classics above says that our thoughts must include what lies beyond the senses as well as what lies within them, and the literal meaning of the word “spirit” points to the central role that breath plays in perceptions and sensations of such a nature.
The phrase “if there is ch’i, then there is no strength” refers to the effortlessness of posture that can be realized in the natural movement of breath, an effortlessness that is based on the simultaneity of equalibrioception (that sustains the posture) and proprioception/graviception (that supports it).
The phrase “without ch’i then essential hardness is achieved” refers to the effort involved in posture when some aspect of posture is not supported; the body will develop strength as necessary to compensate for the lack of support.
I’m never happier than when simple mindfulness of just breathing in or breathing out occurs in me. That I find a natural mindfulness of breathing in or breathing out in the distinction of the senses and the recollection of the elements that constituted Gautama’s way of living, gives me hope that his way of living is indeed “something perfect in itself” (2).
☞ PDF, A Natural Mindfulness
2) SN V 320-322, Pali Text Society SN V, “Mahavagga” Book X Chapter LIV, pg 285