There is a difference between inhabiting your body and merging with--perhaps 'merging' is not the right word. 'Inhabiting' strikes me as being a squatter within the structure of your own body: looking out through the eye sockets, taking in through the other senses. Using the body as a vehicle and waiting for the next life, I suppose. Or just passing the time.
I suspect it is my weak 'core' muscles that have felt like the missing keystone. This would be an inevitable result of giving birth to two children--all of the strictures of the abdominal basket distorted and elongated, the lower back, both spine and muscles, distinctly bowed inward. And nothing quite ever returning the midsection to its former self. If the muscles at the core go, then the body is unbalanced, bifurcated, and all of its parts move separately in their necessary directions. Some sort of irony that gestating and giving birth to another human being is a destabilizing force. Children do seem, at times, to be a form of chaos.
I'm thinking of Marvell's "A Dialogue between the Soul and Body," with its "Soul inslav'd so many ways" by "bolts of Bones," and so on. The body, of course, with its own unconvincing complaints: the soul as tyrant that animates the body only to its own ends. Some scholars think this four stanza poem is unfinished--only four stanzas long, the body has the last word.
Within faculty psychology, which is early modern psychology, there is a belief that arbitrating between the soul and the body is a third entity: the spirit. At one level, this is another bit of Renaissance equivocation: between two extremes, there lies a mean. At another level, this strikes me as intuitively true. Perhaps today the soul is the provenance of religion, and the body that of medicine, and the two cannot quite meet. But if you look at children with disabilities, for example, even those who lack speech, something other than religious belief inspires many parents to feel that their child is fully "there" in a way that neurologists often find hard to correlate with their understanding of neurophysiology. The worst neurologists chalk it up to parent fantasy; the best understand that what they cannot see or quantify or graph or correlate may simply be outside the field of what they study.
Which brings me back to yoga. If you are a religious person and you believe that people have souls, you might see disability as a body's husk in which a soul does lurk, although it cannot express itself. You might be willing to think like a neurologist: a division exists between soul and body. But if you are like me, religious, but in a kind of intemperate and atavistic and meandering sort of way, you might understand that a spirit bridges the gap, no matter how imperfectly, between body and soul, and that what we see of someone, what we see of their humanity, is the work of this animating force: dim or bright, uneven, inconstant, or steadily blinking. To inhabit the body with intention is, perhaps, to feel the work of your spirit.