Metaphysically speaking, whole of the existence can be viewed through the prism of three basic entities namely jivatman, the embodied soul or the self ; Pramatman, the Supersoul ; and prakriti, the nature. The origin and movements of the jivatman have been described in shlokas 15.7 to 15.9 of the Bhagavad Gita. It has been stated that an eternal fraction of the Supreme Lord having become a living soul acquires five senses and the mind from the nature. It, thus, becomes an embodied soul. That is how the beings are created. It has further been stated that the embodied soul experiences life through body and the mind. When he leaves the body and takes another, he carries these senses and the mind along, in the same manner as the air carries aromas from one place to another.
In other words, the embodied soul is an inalienable part of Ishvara, the God. Its distinctiveness is determined by the senses and the mind which it draws, from nature, to itself. The jivatman forms an inseparable part of the Supreme Lord in the same manner as the space in an earthen pot, though identifiable separately, is an inseparable part of the universal space. The embodied soul is qualitatively the same as the Supreme Lord, just as pieces of the gold are also gold.
The jivatman, thus, expresses one part of the all-pervading consciousness. The Divine does not move from place to place. It is the embodied soul which travels from one abode to another. Each time it takes birth, it acquires a body-mind complex out of the materials available in nature, as per his past evolution and future needs.
Beings manifest from the union of the material and the consciousness natures of Ishvara, the Supreme Lord (shlokas 7.4 to 7. 6). The integral and indivisible reality of the Divine appears to be divided into multiplicity of beings (Shloka 13.17).
Thus, it is evident that the mind-body complex and the soul are different. While the former is perishable, the latter is eternal and imperishable. The jivatman does not cease to exist when the body ceases to be. It is the inner light that continues to shine from birth to birth and from life to life.
The self is superior to everything else in the human being. It is stated in shloka 3.42 that senses are great, but mind is greater than the senses ; and intelligence is greater than the mind. And that which is greater than even the intelligence, is the self.
Further, nature of the jivatman has been explained in shlokas from 2.17 to 2.25. As per these shlokas Atman (or jivatman) is all-pervading, indestructible, immutable, eternal and incomprehensible. He is unmanifest, unthinkable ; and unchanging. He can neither kill anyone nor can anyone kill him. He does not get killed when the body is killed. Neither is he ever born, nor does he die at any time. Having once come into being, he never ceases to be. He is unborn, ever-existing and primeval. He can neither be cut by any weapon nor can he be burned by fire. Waters do not make him wet, nor does wind make him dry. He is indivisible. He is the same for ever.
Just as a person discards worn-out garments and puts on new ones ; in the same manner, the embodied soul gives up old bodies and takes up new ones.
Gita recognises the fact that it is not very easy for an ordinary human being to gauge the true nature of the soul. That is why it is stated that some people look upon soul with amazement, others speak about it with amazement ; and still others hear about it with amazement. But even after hearing about him, no one can fathom his mystery (Shloka 2.29). Shlokas 15.10 and 15.11 have also re-iterated this view wherein it is stated that the unintelligent and the deluded ones cannot see the indwelling soul when he is staying, experiencing or departing from the body. Only the ones who have an eye of wisdom can.
The concept of the body, the embodied soul and the Supreme Lord has been explained in a slightly different terminology in the chapter 13. The physical body, mind and the entire material world is called the field (kshetra). The one who is aware of the field is kshetragya (the knower of the field). As the one sun illumines the entire universe, so does the knower of the field (Atman) illuminate (through consciousness) this field (the entire body). All beings are born from the union of the field and the knower of the field. The Supreme Lord is the kshetragya in all the fields.
When the self is associated with the gunas, it is called kshetragya. And when it is released from these modes of material nature, it is called Pramatman or the Supersoul.
In this very chapter, the physical nature and the soul have been called prakriti and purusha respectively. Both prakriti and purusha are beginningless. Whereas the physical nature is the reason behind all material causes and their effects ; the embodied soul is the reason behind all experiences of pleasure and pain.
A word of caution here. The thoughts, emotions, perceptions, intuition, logic and will etc. should not be confused with the soul. All these are the functions of body, mind and the intelligence. Jivatman is not the doer but only a witness.