There are many transcendentalists on the path of spiritual self-realization, but one who is not situated in self-realization cannot see how things are changing in the body of the living entity. The word yoginaḥ is significant in this connection. In the present day there are many so-called yogīs, and there are many so-called associations of yogīs, but they are actually blind in the matter of self-realization. They are simply addicted to some sort of gymnastic exercise and are satisfied if the body is well built and healthy. They have no other information. They are called yatanto 'py akṛtātmānaḥ. Even though they are endeavoring in a so-called yoga system, they are not self-realized. Such people cannot understand the process of the transmigration of the soul. Only those who are actually in the yoga system and have realized the self, the world, and the Supreme Lord—in other words, the bhakti-yogīs, those engaged in pure devotional service in Kṛṣṇa consciousness—can understand how things are taking place.
(Bhagavad-Gita----15:11----purport).

Fruitive work which has perpetually engaged the living being is compared to the banyan tree in the Bhagavad-gītā, for it is certainly very deeply rooted. As long as the propensity for enjoying the fruit of work is there, one has to continue the transmigration of the soul from one body or place to another, according to one's nature of work. The propensity for enjoyment may be turned into the desire for serving the mission of the Lord, By doing so, one's activity is changed into karma-yoga, or the way by which one can attain spiritual perfection while engaging in the work for which he has a natural tendency. Here the word ātmā indicates the categories of all fruitive work. The conclusion is that when the result of all fruitive and other work is dovetailed with the service of the Lord, it will cease to generate further karma and will gradually develop into transcendental devotional service, which will not only cut off completely the root of the banyan tree of work but will also carry the performer to the lotus feet of the Lord.
(Srimad Bhagavatam----1:5:34----purport).

Constant transmigration of the soul from one body to another is the cause of suffering in material existence. This conditional life in material existence is called saṁsāra. One may perform good work and take his birth in a very nice material condition, but the process under which birth and death take place is like a terrible fire. Śrī Viśvanātha Cakravartī Ṭhākura, in his prayer to the spiritual master, has described this. Saṁsāra, or the repetition of birth and death, is compared to a forest fire. A forest fire takes place automatically, without anyone's endeavor, by the friction of dried wood, and no fire department or sympathetic person can extinguish it. The raging forest fire can be extinguished only when there is a constant downpour of water from a cloud. The cloud is compared to the mercy of the spiritual master. By the grace of the spiritual master the cloud of the mercy of the Personality of Godhead is brought in, and then only, when the rains of Kṛṣṇa consciousness fall, can the fire of material existence be extinguished. This is also explained here. In order to find freedom from the stereotyped conditional life of material existence, one has to take shelter of the lotus feet of the Lord, not in the manner in which the impersonalists indulge, but in devotional service, chanting and hearing of the activities of the Lord. Only then can one be freed from the actions and reactions of material existence.
(Srimad Bhagavatam----3:21:17----purport).

The time factor cannot affect the span of life of the devotees. In Bhagavad-gītā it is stated that a little execution of devotional service saves one from the greatest danger. The greatest danger is transmigration of the soul from one body to another, and only devotional service to the Lord can stop this process. It is stated in the Vedic literatures, hariṁ vinā na mṛtim taranti: without the mercy of the Lord, one cannot stop the cycle of birth and death. In Bhagavad-gītā it is stated that only by understanding the transcendental nature of the Lord and His activities, His appearance and disappearance, can one stop the cycle of death and go back to Him.
(Srimad Bhagavatam----3:21:18----purport).

An important word in this verse is mukta-liṅgaḥ. Mukta means "liberated," and liṅga means "the subtle body." When a man dies, he quits the gross body, but the subtle body of mind, intelligence and ego carries him to a new body. While existing in the present body, the same subtle body carries him from one stage of life to another (for example, from childhood to boyhood) by mental development. The mental condition of a baby is different from that of a boy, the mental condition of a boy is different from that of a young man, and the mental condition of a young man is different from that of an old man. So at death the process of changing bodies takes place due to the subtle body; the mind, intelligence and ego carry the soul from one gross body to another. This is called transmigration of the soul. But there is another stage, when one becomes liberated even from the subtle body; at that time the living entity is competent and fully prepared to be transferred to the transcendental or spiritual world.
(Srimad Bhagavatam----4:12:18----purport).

Actually the living entity is in the subtle body, even though the gross body apparently changes and even though he inhabits the gross body on various planets. All the activities performed by the living entity in the subtle body are called illusory because they are not permanent. Liberation means getting out of the clutches of the subtle body. Liberation from the gross body simply involves the transmigration of the soul from one gross body to another. When the mind is educated in Kṛṣṇa consciousness, or higher consciousness in the mode of goodness, one is transferred either to the upper, heavenly planets or to the spiritual world, the Vaikuṇṭha planets. One therefore has to change his consciousness by cultivating knowledge received from Vedic instructions from the Supreme Personality of Godhead through the disciplic succession.
(Srimad Bhagavatam----4:29:61----purport).

The activities of the living entity in the body of a dog may be experienced in the mind of a different body; therefore those activities appear never to have been heard or seen. The mind continues, although the body changes. Even in this life-span we can sometimes experience dreams of our childhood. Although such incidents now appear strange, it is to be understood that they are recorded in the mind. Because of this, they become visible in dreams. The transmigration of the soul is caused by the subtle body, which is the storehouse of all kinds of material desires. Unless one is fully absorbed in Kṛṣṇa consciousness, material desires will come and go. That is the nature of the mind—thinking, feeling and willing. As long as the mind is not engaged in meditation on the lotus feet of the Supreme Personality of Godhead, Kṛṣṇa, the mind will desire so many material enjoyments.
(Srimad Bhagavatam----4:29:68----purport).

"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. In these matters we have to take the authorized statements of Bhagavad-gītā (2.13):
dehino 'smin yathā dehe
kaumāraṁ yauvanaṁ jarā
tathā dehāntara-prāptir
dhīras tatra na muhyati
"As the embodied soul continually passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. The self-realized soul is not bewildered by such a change." Unless all human society understands this important verse in Bhagavad-gītā, civilization will advance in ignorance, not in knowledge.
(Srimad Bhagavatam----4:29:75----purport).