The soul is atomic in size and can be perceived by perfect intelligence. This atomic soul is floating in the five kinds of air (prāṇa, apāna, vyāna, samāna and udāna), is situated within the heart, and spreads its influence all over the body of the embodied living entities. When the soul is purified from the contamination of the five kinds of material air, its spiritual influence is exhibited."
The haṭha-yoga system is meant for controlling the five kinds of air encircling the pure soul by different kinds of sitting postures—not for any material profit, but for liberation of the minute soul from the entanglement of the material atmosphere. So the constitution of the atomic soul is admitted in all Vedic literatures, and it is also actually felt in the practical experience of any sane man. Only the insane man can think of this atomic soul as all-pervading viṣṇu-tattva.
The influence of the atomic soul can be spread all over a particular body. According to the Muṇḍaka Upaniṣad, this atomic soul is situated in the heart of every living entity, and because the measurement of the atomic soul is beyond the power of appreciation of the material scientists, some of them assert foolishly that there is no soul. The individual atomic soul is definitely there in the heart along with the Supersoul, and thus all the energies of bodily movement are emanating from this part of the body. The corpuscles which carry the oxygen from the lungs gather energy from the soul. When the soul passes away from this position, the activity of the blood, generating fusion, ceases. Medical science accepts the importance of the red corpuscles, but it cannot ascertain that the source of the energy is the soul. Medical science, however, does admit that the heart is the seat of all energies of the body.
(Bhagavad-Gita-----2:17-----purport).

There are two kinds of souls—namely the minute particle soul (aṇu-ātmā) and the Supersoul (vibhu-ātmā). This is also confirmed in the Kaṭha Upaniṣad (1.2.20) in this way:
aṇor aṇīyān mahato mahīyān
ātmāsya jantor nihito guhāyām
tam akratuḥ paśyati vīta-śoko
dhātuḥ prasādān mahimānam ātmanaḥ
"Both the Supersoul [Paramātmā] and the atomic soul (jīvātmā) are situated on the same tree of the body within the same heart of the living being, and only one who has become free from all material desires as well as lamentations can, by the grace of the Supreme, understand the glories of the soul." Kṛṣṇa is the fountainhead of the Supersoul also, as it will be disclosed in the following chapters, and Arjuna is the atomic soul, forgetful of his real nature; therefore he requires to be enlightened by Kṛṣṇa, or by His bona fide representative (the spiritual master).
(Bhagavad-Gita-----2:20-----purport).

Change of body by the atomic individual soul is an accepted fact. Even the modern scientists who do not believe in the existence of the soul, but at the same time cannot explain the source of energy from the heart, have to accept continuous changes of body which appear from childhood to boyhood and from boyhood to youth and again from youth to old age. From old age, the change is transferred to another body. This has already been explained in a previous verse (2.13). Transference of the atomic individual soul to another body is made possible by the grace of the Supersoul. The Supersoul fulfills the desire of the atomic soul as one friend fulfills the desire of another. The Vedas, like the Muṇḍaka Upaniṣad, as well as the Śvetāśvatara Upaniṣad, compare the soul and the Supersoul to two friendly birds sitting on the same tree. One of the birds (the individual atomic soul) is eating the fruit of the tree, and the other bird (Kṛṣṇa) is simply watching His friend. Of these two birds—although they are the same in quality—one is captivated by the fruits of the material tree, while the other is simply witnessing the activities of His friend. Kṛṣṇa is the witnessing bird, and Arjuna is the eating bird. Although they are friends, one is still the master and the other is the servant. Forgetfulness of this relationship by the atomic soul is the cause of one's changing his position from one tree to another, or from one body to another. The jīva soul is struggling very hard on the tree of the material body, but as soon as he agrees to accept the other bird as the supreme spiritual master—as Arjuna agreed to do by voluntary surrender unto Kṛṣṇa for instruction—the subordinate bird immediately becomes free from all lamentations.
(Bhagavad-Gita-----2:22-----purport).

This individual soul is unbreakable and insoluble, and can be neither burned nor dried. He is everlasting, present everywhere, unchangeable, immovable and eternally the same.
All these qualifications of the atomic soul definitely prove that the individual soul is eternally the atomic particle of the spirit whole, and he remains the same atom eternally, without change. The theory of monism is very difficult to apply in this case, because the individual soul is never expected to become one homogeneously. After liberation from material contamination, the atomic soul may prefer to remain as a spiritual spark in the effulgent rays of the Supreme Personality of Godhead, but the intelligent souls enter into the spiritual planets to associate with the Personality of Godhead.
The word sarva-gata ("all-pervading") is significant because there is no doubt that living entities are all over God's creation. They live on the land, in the water, in the air, within the earth and even within fire. The belief that they are sterilized in fire is not acceptable, because it is clearly stated here that the soul cannot be burned by fire. Therefore, there is no doubt that there are living entities also in the sun planet with suitable bodies to live there. If the sun globe is uninhabited, then the word sarva-gata—"living everywhere"—becomes meaningless.
(Bhagavad-Gita-----2:24----translation and purport).

It is said that the soul is invisible, inconceivable and immutable. Knowing this, you should not grieve for the body. As described previously, the magnitude of the soul is so small for our material calculation that he cannot be seen even by the most powerful microscope; therefore, he is invisible. As far as the soul's existence is concerned, no one can establish his existence experimentally beyond the proof of śruti, or Vedic wisdom. We have to accept this truth, because there is no other source of understanding the existence of the soul, although it is a fact by perception. There are many things we have to accept solely on grounds of superior authority. No one can deny the existence of his father, based upon the authority of his mother. There is no source of understanding the identity of the father except by the authority of the mother. Similarly, there is no source of understanding the soul except by studying the Vedas. In other words, the soul is inconceivable by human experimental knowledge. The soul is consciousness and conscious—that also is the statement of the Vedas, and we have to accept that. Unlike the bodily changes, there is no change in the soul. As eternally unchangeable, the soul remains atomic in comparison to the infinite Supreme Soul. The Supreme Soul is infinite, and the atomic soul is infinitesimal. Therefore, the infinitesimal soul, being unchangeable, can never become equal to the infinite soul, or the Supreme Personality of Godhead. This concept is repeated in the Vedas in different ways just to confirm the stability of the conception of the soul. Repetition of something is necessary in order that we understand the matter thoroughly, without error.
(Bhagavad-Gita-----2:25----translation and purport).

Even if Arjuna did not believe in the existence of the soul—as in the vaibhāṣika philosophy—there would still have been no cause for lamentation. No one laments the loss of a certain bulk of chemicals and stops discharging his prescribed duty. On the other hand, in modern science and scientific warfare, so many tons of chemicals are wasted for achieving victory over the enemy. According to the vaibhāṣika philosophy, the so-called soul or ātmā vanishes along with the deterioration of the body. So, in any case, whether Arjuna accepted the Vedic conclusion that there is an atomic soul or he did not believe in the existence of the soul, he had no reason to lament. According to this theory, since there are so many living entities generating out of matter every moment, and so many of them are being vanquished every moment, there is no need to grieve for such incidents. If there were no rebirth for the soul, Arjuna had no reason to be afraid of being affected by sinful reactions due to his killing his grandfather and teacher. But at the same time, Kṛṣṇa sarcastically addressed Arjuna as mahā-bāhu, mighty-armed, because He, at least, did not accept the theory of the vaibhāṣikas, which leaves aside the Vedic wisdom. As a kṣatriya, Arjuna belonged to the Vedic culture, and it behooved him to continue to follow its principles.
(Bhagavad-Gita-----2:26---- purport).

The fact that the atomic soul is within the body of a gigantic animal, in the body of a gigantic banyan tree, and also in the microbic germs, millions and billions of which occupy only an inch of space, is certainly very amazing. Men with a poor fund of knowledge and men who are not austere cannot understand the wonders of the individual atomic spark of spirit, even though it is explained by the greatest authority of knowledge, who imparted lessons even to Brahmā, the first living being in the universe. Owing to a gross material conception of things, most men in this age cannot imagine how such a small particle can become both so great and so small. So men look at the soul proper as wonderful either by constitution or by description. Illusioned by the material energy, people are so engrossed in subject matters for sense gratification that they have very little time to understand the question of self-understanding, even though it is a fact that without this self-understanding all activities result in ultimate defeat in the struggle for existence. Perhaps they have no idea that one must think of the soul, and thus make a solution to the material miseries.
(Bhagavad-Gita-----2:29---- purport).

Some people who are inclined to hear about the soul may be attending lectures, in good association, but sometimes, owing to ignorance, they are misguided by acceptance of the Supersoul and the atomic soul as one without distinction of magnitude. It is very difficult to find a man who perfectly understands the position of the Supersoul, the atomic soul, their respective functions and relationships and all other major and minor details. And it is still more difficult to find a man who has actually derived full benefit from knowledge of the soul, and who is able to describe the position of the soul in different aspects. But if, somehow or other, one is able to understand the subject matter of the soul, then one's life is successful. The easiest process for understanding the subject matter of self, however, is to accept the statements of the Bhagavad-gītā spoken by the greatest authority, Lord Kṛṣṇa, without being deviated by other theories. But it also requires a great deal of penance and sacrifice, either in this life or in the previous ones, before one is able to accept Kṛṣṇa as the Supreme Personality of Godhead. Kṛṣṇa can, however, be known as such by the causeless mercy of the pure devotee and by no other way.
(Bhagavad-Gita-----2:29---- purport).

In the Muṇḍaka Upaniṣad (3.1.9) the situation of the soul in relation to prāṇa is explained:
eṣo 'ṇur ātmā cetasā veditavyo
yasmin prāṇaḥ pañcadhā saṁviveśa
prāṇaiś cittaṁ sarvam otam prajānāṁ
yasmin viśuddhe vibhavaty eṣa ātmā
"The soul is atomic in size and can be perceived by perfect intelligence. This atomic soul is floating in the five kinds of air [prāṇa, apāna, vyāna, samāna and udāna]. The soul is situated within the heart, and it spreads its influence all over the body of the embodied living entities. When the soul is purified from the contamination of the five kinds of material air, its spiritual influence is exhibited." Thus in the innumerable species of life the spiritual soul remains situated within prāṇa, or the material life air.
(Srimad Bhagavatam-----11:3:39-----purport).