There are three bodies, viz., gross body, subtle body and causal body. The bodies and Koshas can have no existence independent of Atman. That which is seen by the physical eyes, that which is composed of flesh, bones, fat, skin, nerves, hair, blood, etc., is the physical body. It has six changes, viz., birth, existence, growth, modification, decay and death. This body grows in youth and decays in old age. It develops when good nourishing food is given, and decays if food is withdrawn, or if there is some disease. Because it disintegrates and decays, it is called Sarira. As the body is burnt after death, it is called Deha. The subtle body is composed of seventeen Tattvas, viz., five Jnana Indriyas, five Karma Indriyas, five Pranas, mind and intellect. The subtle body develops through intense Abhimana or egoism and strong Raga-Dvesha, and decays when egoism and Raga-Dvesha are destroyed, or decreased. The subtle body is burnt by the three kinds of Tapas, viz., Adhyatmic, Adhibhautic and Adhidaivic. Ajnana or Avidya alone constitutes the Karana Sarira. The causal body develops through the idea, I am Jiva and decays when this idea is annihilated or decreased, when 'I' is identified with Brahman. The subtle and causal bodies are thickened in worldly-minded persons and are thinned out in earnest aspirants.
The fire of wisdom (Jnanagni) completely destroys the subtle and causal bodies. Subtle body is also called Linga Sarira as it enables one to have the experience of hearing, feeling, seeing, tasting and smelling and to realise the Atman through meditation. That which is the cause for the gross and subtle bodies is called Karana Sarira or causal body.
We can clearly see the physical body. As we cannot see the subtle body, it is sometimes asked how can we say that the subtle body exists? The existence of the subtle body can be known or inferred from the effects caused by the seventeen organs. Why can we not say that these seventeen kinds of actions are done by the physical body? In deep sleep, swoon and death, these seventeen kinds of actions are not done by the physical body. Therefore we cannot say that these belong to the physical body. Therefore, we should admit that a subtle body endowed with seventeen faculties does exist irrespective of the physical body.
An objector says: The gross and subtle bodies do not work separately. They both conjointly do the functions of hearing, seeing, etc. If you reflect well, you will find that the subtle body does all the functions, and not both together. An illustration will clear this point well. The fire cooks food and does other actions with the help of the fuel, and yet the act of cooking is due to the fire only and not due to the fuel. Even so the actions of seeing, hearing, etc., that are done by the subtle body which depends upon the gross body for its operations are due to the subtle body and not due to the gross body. Is it clear now? Therefore, it must be clearly admitted beyond a shadow of doubt that the subtle body with seventeen faculties does exist.
The physical body is inert. It has a beginning and an end. It is full of impurities. It is perishable. It is a product of five elements. It is full of parts. It can be seen by the naked eye. It is finite (Parischinna). It is changing. So, it cannot be self-existent, self-luminous, and Satchidananda.
The subtle body is also inert. It has a beginning and an end. It is a product of five Tanmatras or subtle elements. It is changing. It is perishable. It is also finite. This cannot be the ever pure, intelligent Atman.
The causal body is nothing but Ajnana or primitive ignorance. It is also inert. Man says: I am a person of Ajnana. Though it is beginningless, it has an end. It is death for the causal body when one gets Knowledge of the Self. This cannot be the eternal, pure, indivisible, infinite Atman.
The Atman is entirely distinct from the three bodies. He who has realised his Atman and who knows that he is entirely distinct from the three bodies is an emancipated person. He has crossed this ocean of Samsara. He is Brahman Himself. He is adored in the three worlds. His glory is indescribable.