The psychic Prana acts as something of a lynch-pin between the physical body and the mental being. Sri Aurobindo describes the issues that arise: “The psychical Prana interferes in all the higher operations to deform them, but its defect is itself due to its being interfered with and deformed by the nature of the physical workings in the body which Life has evolved in its emergence from Matter. It is that which has created the separation of the individual life in the body from the life of the universe and stamped on it the character of want, limitation, hunger, thirst, craving for what it has not, a long groping after enjoyment and a hampered and baffled need of possession.”
The action of the life-force in the body is self-regulating in the physical realm. There is a natural balance between species, and even a symbiotic relationship whereby plants and animals depend upon one another to survive and thrive and thus, achieve homeostasis. The force of hunger, for instance, works itself out in the animal kingdom to the extent that large predators only eat what they need to survive and they generally will not undertake to decimate a population of their prey. If things were to get temporarily out of balance, due to a large increase in the predator population, the massive decrease of natural prey that results would restore the balance within a generation or two.
When the psychic Prana however brings the motivations of hunger, thirst, lust, etc. into the mental realm, however, imbalance quickly becomes apparent. “Easily regulated and limited in the purely physical order of things, it extends itself in the psychical Prana immensely and becomes, as the mind grows, a thing with difficulty limited, insatiable, irregular, a busy creator of disorder and disease.”
There is another issue as well: “Moreover, the psychical Prana leans on the physical life, limits itself by the nervous force of the physical being, limits thereby the operations of the mind and becomes the link of its dependence on the body and its subjection to fatigue, incapacity, disease, disorder, insanity, the pettiness, the precariousness and even the possible dissolution of the workings of the physical mentality. Our mind instead of being a thing powerful in its own strength, a clear instrument of conscious spirit, free and able to control, use and perfect the life and body, appears in the result a mixed construction; it is a predominantly physical mentality limited by its physical organs and subject to the demands and to the obstructions of the life in the body.”
The apparent solution to these problems of the psychic Prana is for the seeker to identify with the mind and eventually come to see it as separate from the bodily life. The seeker then can begin to develop the pure power of the mentality and reduce the reliance on the physical and vital instruments for its processes. This makes the life and body “…a transmitting channel for the Idea and Will in the Buddhi, obedient to its suggestions and commands; the Prana then becomes a passive means of effectuation for the mind’s direct control of the physical life….This control can be exercised perfectly, however, only from the supramental level, for it is there that the true effective Idea and Will reside and the mental thought-mind, even spiritualised, is only a limited, though it may be made a very powerful deputy.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 6, Purification–the Lower Mentality, pp. 630-631