A soul resides in the gross body that is made of physical elements and it performs a number of karma. Even after their conclusion, the karma leaves an impression within resulting in our rebirth. After death, the impressions go along with the soul as carried by the subtle body. These impressions are responsible for the future births of the soul. Indian thinkers have developed elaborate philosophy in this direction. Upanishads and other books of philosophy deal with these questions in detail. The gist of the same appears in the ninth chapter of Satyārtha-Prakāśa by Maharṣi Dayānanda: “There are three bodies. First is the gross physical body that is visible to us. The second one consists of five sense organs, five vital life forces, five subtle elements, mind and Buddhi – total 17 elements, called Subtle Body. The subtle body remains intact with the soul even after death and follows it to the rebirth. This has two parts – the physical part of the subtle elements, and the consequential part that has the attributes specific to the particular soul. The second part of the subtle body remains with the soul even during its salvation wherein it enjoys pure bliss. Third is the causal body wherein the soul resides during the deep sleep and that is uniformly same for all the souls being the primordial state of matter.”
Five Vital Life Forces (Prāṇa): The subtle body consists of five vital life forces –
Prāṇa – that inspires/inducts things inward from outside.
Apāna – that expires/expels outward from within.
Samāna – stationed in the navel region and distributes juices/essence throughout the body.
Udāna – that inducts food and liquids from the throat region and exercises force outwardly.
Vyāna – that causes various efforts made by different body organs.
Note: Swami Dayanand Saraswati has described Prana as the exhalation and Apana as the inhalation. That may be an error of some sort. Different scholars have different opinions about this issue.
Five Organs for Knowledge: The organs for sight, sound, smell, taste and touch reside in the subtle body and are different from the outer visible organs such as eyes, ears, nose, tongue, and skin, which merely pick up the knowledge signals from the outer world, but, the inner organs ultimately convey the signals to the Manas.
Five Subtle elements, Mind and Buddhi: Sound, touch, sight, taste and smell are the five subtle elements or five tan-mātrās . They are located in the subtle body.
Thus, five Prāṇa, five organs for knowledge, five subtle elements, Manas and Buddhi constitute 17 elements of the Subtle Body. This is the physical part of the subtle body. The non-physical subtle body constitutes the inherent properties of the soul, which exist in salvation too.
Four-Fold Inner Instrument (Antaḥ-Karaṇa Catuṣtaya): Karaṇa means instrument. Thus, antaḥ-karaṇa would mean the inner instruments within the body. Based on the functional divisions, there are four divisions – Manas(मन), Buddhi(बुद्धि), Citta(चित्त) and Ahaṁkāra(अहंकार). Their functions are: to think different alternatives, to decide, to memorise, and to give an impression of the existence of the self. Manas does the thinking operation, weighing different options and evaluates different possibilities. Buddhi has the discriminatory ability and concludes the thinking operation to an ultimate decisive point. Out of what transpires in mind, one option is finally decided upon by the soul using the discriminatory ability of the Buddhi. The third element of the inner instruments is Citta. All that an individual has performed as karma, remains stored in the memory and that leaves an impression on the Citta. Depending upon the requirement, the soul retrieves the same from its memory. The fourth element is Ahaṁkāra, i.e. to know the ones self. To know one self and as different from other things is the function of Ahaṁkāra.
Mind, Prāṇa and Organs: Prāṇa exist in between the sense organs and mind. The Prāṇa is under the control of the mind. When the soul desires to establish the contact with the outer world then through the mind, it activates the Prāṇa and then the sense organs develop contact with the outer world. Thus, it is primarily the soul that intends to get connected to the outer world. It instructs the mind to direct the Prāṇa. Prāṇa, in turn direct the sense organs and the latter get connected to the outside world.
The worldly objects do not get connected to the mind directly; rather they do so through the sense organs. The sense organs bring in the incoming knowledge from the objects they have perceived and convey the same to the mind through the network of nerves. The mind then evaluates all possible options and the intellect draws a conclusion with respect to the same. In the process, the memory plays an important role through the impressions stored therein.
For example, the eyes come across a human being. That information enters into the mind. The mind then evaluates several possible options that the particular person could be. The particular person was seen some time ago too. The previous impressions are stored in the memory and they get revived and with their help, the intellect concludes that the presently appearing person is such and such person seen earlier on such an occasion. [The topic has been discussed at length in the ninth chapter of Satyārtha-Prakāśa and in other books of Indian philosophy.]