Before we move on from this topic, let me add a few more thoughts by way of summing up.
The nervous system is the body’s primary mechanism for optimizing movement, and “mind” is a catch-all term for those of the brain’s multifarious communicative activities that we can know or infer. Once the decision to sit and immobilize the body has been made, the amount of sensory input falls off gradually. Merely by sitting still, signals from the environment and signals from within the organism itself are greatly reduced, and the volume of conscious perceptions continue to decrease by stages as the ability to remain focused on somatic processes improves. The bulk of what is experienced during quiet sitting comprises ambient noise, bodily sensations, and a range of unsolicited thoughts.
As the forgoing sketch of its components suggests, the regulation of posture, although fairly straightforward in method, is complex and rather subtle. It is fruitful and interesting because in order simply to sit up straight and relax we are required, first, to become aware of processes that are normally habitual and therefore unconscious and, second, to gain an extraordinary measure of sensitivity to and control over the disposition of the body in space.
That undertaking entails the gathering and processing of various kinds of sensory information from multiple sources and projecting them upon a conceptual-sensory map of the body. The range of data so processed includes inputs from the skin, from a great many muscle groups, and even from the viscera: pressure, tension, bodily orientation, and clusters of what psychologists call interocepts, signals that keep us informed of the status of the systems that regulate our physiological processes. In the early stages of practice we are sensitive to ordinary tactile sensation and the grosser proprioceptive phenomena. By degrees we become able to discern the slightest changes in heartbeat, rate of breathing, and bodily structure.
The development of posture is fundamental to the practice of seated meditation because by attending to the sensations and physiological activities of the body the trainee can come to see that one’s sense of bodily identity is constructed, variable, and transient. Liberating insights of this kind can arise without any special instruction in techniques of contemplation.
With the regular practice of quiet sitting, the trainee can experience deep relaxation, a foretaste of freedom through the calming of thought, increased ability to notice and accept change, and –best of all—escape from the tyranny of urges, cravings, and moods.