From the book "Jap Ji – The message of Guru Nanak", written by Sant Kirpal Singh
(1) Spirit or the soul, represents the region of the spiritual division.
(2) Mind or the mental plane, refers to that of the universal mind.
(3) Physical body or the material planes, consist of three bodies: the causal, the astral and the gross.
The gross body is the fleshy sheath we have. It consists of the gross matter and gross organs of actions and senses: eyes, ears, nose, tongue, skin and excretory and generative organs. It is shaken off at the time of physical death.
The astral body consists of subtle matter and subtle organs and is active in dreamland state. In this is located the mind.
The causal body is the root cause of the other two bodies. Its activity is confined to the deep sleep state. The latter two bodies along with the mind continue to exist after death and create a new form or physical frame at each rebirth.
Macrocosm thus exists in the microcosm of the human body. The knowledge of the latter serves as means to the comprehension of the former. If one could succeed in attaining the Region of the Spirit within himself, by shaking off these sheaths, he can cast off all pain and misery and enjoy incessant bliss and peace untold. A life of matter is all misery which one cannot escape until one is able to separate oneself from it.
The attainment of the region of spirit then is the thing that is required, for it can lead to the abode of perpetual joy and supreme happiness, even while living in this physical body. It is only by riding the current of the Holy Naam, (the eternal Music in man), that this state is reached, with the help of the True Master. The Holy Naam – the Word – comes down from the highest spiritual planes, the region of Truth, into the material planes. It is the saving lifeline that can carry souls into the plane beyond the reach of dissolution. All other means fall short of the goal.
The kingdom of God is within you; you may seek it there. It is within your finite body that communion with the Word can be held. This ultimately brings you to the Region of Infinite Truth – the Sach Khand.