Kṛṣṇa says in the Bhagavad-gītā,
- dehino 'smin yathā dehe
- kaumāraṁ yauvanaṁ jarā
- tathā dehāntara-prāptir
- dhīras tatra na muhyati
- (BG 2.13)
He is giving very practical example that because the soul is within this body, therefore bodily changes are taking place. Now, suppose if a child is born dead. That. . . You can keep it by some method, preservating method, but it will not grow. One can understand very easily. Because the soul is there, therefore the child from the womb of his mother grows gradually. Grows means changing body. Everything information is there in the Śrīmad-Bhāgavatam. The man and woman, father and mother, after sex the two secretion is emulsified, and it takes a form like a pea. And the small living entity, the measurement is also given, keśāgra-śata-bhāgasya śatadhā kalpitaḥ (CC Madhya 19.140): one ten-thousandth part of the upper portion point of hair. You know the point of hair. Now divide it into ten thousand part and take one part. That is the form of the soul. That little spark takes shelter into that emulsified pea, and because the soul is there within, it develops from the mother's womb. The child does not develop all of a sudden. Every mother knows that. It grows gradually, little by little, little by little. When it is seven months, then it is further. . . it moves. So in this way the. . . Kṛṣṇa says that don't take this body as the living being. Dehinaḥ asmin dehe. Within this body, the soul is there. So everyone can understand.
Now, because the soul is there, therefore the body is changing. Everyone knows that he was a child. I know; you know. I remember my childhood body or my boyhood body, my youthhood body. I am old man. I remember them, that "I was doing as a young man like this. I was doing as a boy like this. I was jumping. Now I cannot jump. Why?" The body has changed. The body has changed. It is very good logic. The same "I" am there. I was a boy, I was jumping. Now I am old man, I have changed my body, I cannot do that. I will have to take the stick. Because the body has changed. So where is the fallacy of logic? It is very clearly. . . And the authority says. . . Kṛṣṇa says, not an ordinary person. He says, "Within this body there is the soul, and as on account of the soul the body is changing shape from childhood to boyhood, boyhood to youthhood, from youthhood to old man's body, similarly, when this body will be useless, he will accept another body."
This simple truth one has to understand first of all before anything spiritual knowledge. If one cannot understand that the spirit soul is different from this body, then he is cat and dog. He is not human being.