Sri Aurobindo has described Yoga as applied psychology. In order to effectuate change in human psychology, the elements of the human psychological structure must be understood and their interaction noted. With this purpose in mind, Sri Aurobindo describes the human instrument and its usual workings:
“Mind, life and body are the three powers of our lower nature. But they cannot be taken quite separately because the life acts as a link and gives its character to body and to a great extent to our mentality. Our body is a living body; the life-force mingles in and determines all its functionings. Our mind too is largely a mind of life, a mind of physical sensation; only in its higher functions is it normally capable of something more than the workings of a physical mentality subjected to life.”
The body and the life-force make up what yogic psychology calls the “gross body”, sthula sarira. The life-force active in the body is called the physical Prana. “This is only the outer instrument, the nervous force of life acting in the form of body with its gross physical organs.”
There is also an inner side to the human being. “This inner instrument is divided by the old system into four powers; citta or basic mental consciousness; manas, the sense mind; buddhi, the intelligence; ahankara, the ego-idea. The classification may serve as a starting-point, though for a greater practicality we have to make certain farther distinctions. This mentality is pervaded by the life-force, which becomes here an instrument for psychic consciousness of life and psychic action on life. Every fibre of the sense mind and basic consciousness is shot through with the action of this psychic Prana, it is a nervous or vital and physical mentality. Even the Buddhi and ego are overpowered by it, although they have the capacity of raising the mind beyond subjection to this vital, nervous and physical psychology. This combination creates in us the sensational desire-soul which is the chief obstacle to a higher human as well as to the still greater divine perfection. Finally, above our present conscious mentality is a secret supermind which is the proper means and native seat of that perfection.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 5, The Instruments of the Spirit, pp. 619-620
Sri Aurobindo Studies 