People have different ideas about who and what they are, seeing themselves as body, mind or spiritual being or any combination of the former. It does not matter how one answer this question personally, as the Alexander Technique deals with the interaction of body and mind.
When we lie in semi supine we can train the Inner observer. We increase our awareness of proprioception observing our body states, and the changes induced by our thinking and the hands of a teacher. Scanning the own body in this way can feel unfamiliar initially, and the disability to sense or release certain areas can lead to frustration.
However, any structural change in the brain takes its time. By attending our body systematically and persistently we change the neural pathways that receive sensory information and send motor control. Table work in semi supine offers a splendid opportunity to develop the mental facility of Inner observer. We can learn focusing our awareness, and inhibiting to judge perceived changes and thoughts that compete for our attention.
Once we learned to stay more focused during semi supine, and to direct our attention to specific parts of our with ease, we can use the Inner Observer easier in situations with a higher level of stimuli. However, parts of our mind, especially the ego, can set up heaps of traps that interfere with the progress.
It helps to remember some key qualities of the Inner observer, when the technique seems not to work at all. The Inner observer simply registers sensations, thoughts, emotions, but does not judge any of these. Curiosity, and not inquisition drives the Inner observer. The Inner observer conducts the experiments accessing the body's primary control.