Foolish people deny the existence of the soul, but it is a fact that when we sleep we forget the identity of the material body and when we awake we forget the identity of the subtle body. In other words, while sleeping we forget the activities of the gross body, and when active in the gross body we forget the activities of sleeping. Actually both states—sleeping and waking—are creations of the illusory energy. The living entity actually has no connection with either the activities of sleep or the activities of the so-called wakened state.
(Srimad Bhagavatam----4:29:71----purport).

Because of our ignorance of the spiritual body, of which we have no experience, we do not know of the activities of the spiritual body, and in ignorance we jump from one false platform to another. We act sometimes in relation to the gross body and sometimes in relation to the subtle body. If, by Kṛṣṇa's grace, we act in our spiritual body, we can transcend both the gross and subtle bodies. In other words, we can gradually train ourselves to act in terms of the spiritual body. As stated in the Nārada-pañcarātra, hṛṣīkeṇa hṛṣīkeśa-sevanaṁ bhaktir ucyate: (CC.2:19:170 devotional service means engaging the spiritual body and spiritual senses in the service of the Lord. When we are engaged in such activities, the actions and reactions of the gross and subtle bodies cease.
(Srimad Bhagavatam----4:29:71----purport).

The activities of the subtle and gross bodies depend on how developed conditions are. The example of the moon is very appropriate. On a dark-moon night, the full shining moon is still present, but it appears not to be present due to conditions. Similarly, the senses of the living entity are there, but they only become active when the gross body and thesubtle body are developed. Unless the senses of the gross body are developed, they will not act on the subtle body. Similarly, because of the absence of desires in the subtle body, there may be no development in the gross body.
(Srimad Bhagavatam----4:29:72----purport).

It is sometimes said that because a child is innocent he is completely pure. Actually this is not the fact. The effects of fruitive activities reserved in the subtle bodyappear in three concurrent stages. One is called bīja (the root), another is called kūṭa-stha (the desire), and another is called phalonmukha (about to fructify). The manifest stage is called prārabdha (already in action). In a conscious or unconscious state, the actions of the subtle or gross bodies may not be manifest, but such states cannot be called the liberated state. A child may be innocent, but this does not mean that he is a liberated soul. Everything is held in reservation, and everything will become manifest in due course of time. Even in the absence of certain manifestations in the subtle body, the objects of sense enjoyment may act. The example has been given of a nocturnal emission, in which the physical senses act even when the physical objects are not manifest. The three modes of material nature may not be manifest in the subtle body, but the contamination of the three modes remains conserved, and in due course of time, it becomes manifest. Even if the reactions of the subtle and gross bodies are not manifest, one does not become free from the material conditions. Therefore it is wrong to say that a child is as good as a liberated soul.
(Srimad Bhagavatam----4:29:73----purport).

The total material existence is first agitated by the three modes of material nature, and these become the living conditions of the living entity. Thus the subtle and gross bodies develop, and the ingredients are earth, water, fire, air, sky, and so on. According to Śrī Madhvācārya, when consciousness, the living force in the heart, is agitated by the three modes of material nature, then the subtle body of the living entity, consisting of the mind, the sense objects, the five senses that acquire knowledge and the five senses for acting in the material condition, becomes possible.
(Srimad Bhagavatam----4:29:74----purport).

As stated in Bhagavad-gītā (4.2):
evaṁ paramparā-prāptam
imaṁ rājarṣayo viduḥ
sa kāleneha mahatā
yogo naṣṭaḥ parantapa
"This supreme science was thus received through the chain of disciplic succession, and the saintly kings understood it in that way. But in course of time the succession was broken, and therefore the science as it is appears to be lost." Guided by so-called psychologists and philosophers, people in the modern age do not know of the activities of the subtle body and thus cannot understand what is meant by the transmigration of the soul.
(Srimad Bhagavatam----4:29:75----purport).

While in the subtle body, we create many plans to enjoy sense gratification. These plans are recorded in the spool of one's mind as bīja, the root of fruitive activities. In conditional life the living entity creates a series of bodies one after another, and this is called karma-bandhana. As explained in Bhagavad-gītā (3.9), yajñārthāt karmaṇo 'nyatra loko 'yam-karma-bandhanaḥ: if we act only for the satisfaction of Viṣṇu, there is no bondage due to material activity, but if we act otherwise, we become entrapped by one material activity after another. Under these circumstances, it is to be supposed that by thinking, feeling and willing, we are creating a series of future material bodies. In the words of Bhaktivinoda Ṭhākura, anādi karama-phale, padi' bhavārṇava jale. The living entity falls into the ocean of karma-bandhana as a result of past material activities.
(Srimad Bhagavatam----4:29:78----purport).

As Śrī Caitanya Mahāprabhu confirmed: ceto-darpaṇa-mārjanam (CC.3:20:12). The more we talk of Kṛṣṇa, think of Kṛṣṇa and preach for Kṛṣṇa, the more we become purified. This means we no longer have to accept a hallucinatory gross and subtle body, but instead attain our spiritual identity. One who tries to understand this instructive spiritual knowledge is delivered from this ocean of nescience. The word pārameṣṭhyam is very significant in this connection. Pārameṣṭhyam is also called Brahmaloka; it is the planet on which Lord Brahmā lives. The inhabitants of Brahmaloka always discuss such narrations so that after the annihilation of the material world, they can be directly transferred to the spiritual world. One who is transferred to the spiritual world does not have to go up and down within this material world. Sometimes spiritual activities are also called pārameṣṭhyam.
(Srimad Bhagavatam----4:29:84----purport).

The body is a symbolic representation of material bondage according to one's karma. Although sometimes we cannot see the gross body changing, chanting the holy name of the Supreme Lord immediately changes the subtle body, and because the subtle body changes, the living entity is immediately freed from material bondage. After all, changes of the gross body are conducted by the subtle body. After the destruction of the gross body, the subtle body takes the living entity from his present gross body to another. In the subtle body, the mind is predominant, and therefore if one's mind is always absorbed in remembering the activities or the lotus feet of the Lord, he is to be understood to have already changed his present body and become purified. Therefore it is irrefutable that a caṇḍāla, or any fallen or lowborn person, can become a brāhmaṇa simply by the method of bona fide initiation.
(Srimad Bhagavatam----5:1:35----purport).

The gross body is a product of the subtle body. As stated in Bhagavad-gītā (8.6):
yaṁ yaṁ vāpi smaran bhāvaṁ
tyajaty ante kalevaram
taṁ tam evaiti kaunteya
sadā tad-bhāva-bhāvitaḥ
"Whatever state of being one remembers when he quits his body, that state he will attain without fail." The atmosphere of the subtle body at the time of death is created by the activities of the gross body. Thus the gross body acts during one's lifetime, and the subtle body acts at the time of death. The subtle body, which is called liṅga, the body of desire, is the background for the development of a particular type of gross body, which is either like that of one's mother or like that of one's father. According to the Ṛg Veda, if at the time of sex the secretions of the mother are more profuse than those of the father, the child will receive a female body, and if the secretions of the father are more profuse than those of the mother, the child will receive a male body. These are the subtle laws of nature, which act according to the desire of the living entity. If a human being is taught to change his subtle body by developing a consciousness of Kṛṣṇa, at the time of death the subtle body will create a gross body in which he will be a devotee of Kṛṣṇa, or if he is still more perfect, he will not take another material body but will immediately get a spiritual body and thus return home, back to Godhead. This is the process of the transmigration of the soul. Therefore instead of trying to unite human society through pacts for sense gratification that can never be achieved, it is clearly desirable to teach people how to become Kṛṣṇa conscious and return home, back to Godhead. This is true now and, indeed, at any time.
(Srimad Bhagavatam----6:1:54----purport).

The conditioned soul has knowledge, and if he wants to fully utilize the gross and subtle bodies for his real advancement in life, he can do so. It is therefore said here that by his high intelligence (svena tejasā), by the superior power of superior knowledge achieved from the right source—the spiritual master, or ācārya—he can give up his conditional life in a material body and return home, back to Godhead. However, if he wants to keep himself in the darkness of this material world, he can do so. The Lord confirms this as follows in Bhagavad-gītā (9.25):
yānti deva-vratā devān
pitṟn yānti pitṛ-vratāḥ
bhūtāni yānti bhūtejyā
yānti mad-yājino 'pi mām
"Those who worship the demigods will take birth among the demigods; those who worship ghosts and spirits will take birth among such beings; those who worship ancestors go to the ancestors; and those who worship Me will live with Me."
(Srimad Bhagavatam----7:2:46----purport).

At the time of death, fire burns the gross body, and if there is no more desire for material enjoyment the subtle body is also ended, and in this way a pure soul remains. This is confirmed in Bhagavad-gītā (tyaktvā dehaṁ punar janma naiti (B.G.4:9). If one is free from the bondage of both the gross and subtle material bodies and remains a pure soul, he returns home, back to Godhead, to be engaged in the service of the Lord. Tyaktvā dehaṁ punar janma naiti mām eti: he goes back home, back to Godhead. Thus it appears that Saubhari Muni attained that perfect stage.
(Srimad Bhagavatam----9:6:54----purport).

The term "spiritual body" is sometimes taken to refer to a ghostly body. An impious man who dies after sinful activities is sometimes condemned so that he cannot possess a gross material body of five material elements, but must live in a subtle body of mind, intelligence and ego. However, as explained in Bhagavad-gītā, devotees can give up the material body and attain a spiritual body free from all material tinges, gross and subtle (tyaktvā dehaṁ punar janma naiti mām eti so 'rjuna (B.G.4:9). Thus the demigods gave King Nimi the benediction that he would be able to stay in a purely spiritual body, free from all gross and subtle material contamination.
(Srimad Bhagavatam----9:13:11----purport).

One should be convinced that he is a spirit soul, part and parcel of the Supreme Brahman, Kṛṣṇa, but has somehow or other been entrapped by the material coverings of the gross and subtle bodies, consisting of earth, water, fire, air, ether, mind, intelligence and false ego. One should know that the association of society, friendship, love, nationalism, religion and so on are nothing but creations of māyā. One's only business is to become Kṛṣṇa conscious and render service unto Kṛṣṇa as extensively as possible for a living being. In this way one is liberated from material bondage. By the grace of Kṛṣṇa, Devayānī attained this state through the instructions of her husband.
(Srimad Bhagavatam----9:19:27-28----purport).

This is the process of the soul's transmigration from one body to another. At the time of death, according to his mental condition, the living being is carried by thesubtle body, consisting of mind, intelligence and ego, to another gross body. When higher authorities have decided what kind of gross body the living entity will have, he is forced to enter such a body, and thus he automatically gives up his previous body. Dull-minded persons who do not have the intelligence to understand this process of transmigration take for granted that when the gross body is finished, one's life is finished forever. Such persons have no brains with which to understand the process of transmigration.
(Srimad Bhagavatam----10:1:40----purport).