So the body is changing. Dehino 'smin yathā dehe kaumāraṁ yauvanaṁ jarā. Kaumāram means boyhood, yauvanam means youthhood, and jarā means old age, aged body. So I can remember—I am an old man—I can remember I had a boy's body, I had a young man's body; now I have got this aged body. So although the boyhood body, the youthhood body are no longer existing, but I am existing. That's a fact. Everyone can understand: he has got past, present and future. You are all young boys and girls present here. So you had your past body as boyhood, childhood; similarly, you have got your future body also. That is awaiting. I have got it; you are awaiting.
So past, future . . . past, present and future, relatively we can understand in any condition of life. Therefore the conclusion is that when this aged body as I have got now . . . I am seventy-seven years old. So when this body will be finished, I'll get another body. As I have got consecutively from boyhood to childhood, childhood, I have . . . from childhood to boyhood, boyhood to youthhood, aged body, so why not next body? This is simple truth, that the living entity, or the soul, is transmigrating from one body to another. This is the basic principle of spiritual understanding.
The vital force of the body is the spirit soul. It is not a mechanical arrangement of matter. The modern so-called scientists, they think that the body is combination of matter and, at a certain stage, these combination of matter develops living symptoms. But that is not a fact. If it is a fact, then the scientists can manufacture with chemicals a living body. But a scientist even up to date is unable to manufacture even a body like an ant, and what to speak of other, bigger animals.
So we have to take this knowledge from authority. Here is Kṛṣṇa speaking. He's authority. We accept Kṛṣṇa: the Supreme Personality of Godhead. His knowledge is perfect. He knows past, present and future. Therefore He is teaching Arjuna, "My dear Arjuna, the spirit soul within this body is eternal." That's a fact. Just like I can understand I was in the past, I am in present, so I must be in the future. These are three phases of time: past, present and future.