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.
In another place we read in this Bhagavad-gītā, na jāyate na mriyate vā kadācit (BG 2.20). The living entity is never born; neither it dies. Na jāyate means it never takes birth. Na jāyate na mriyate, it never dies. Nityaṁ śāśvato 'yam, na hanyate hanyamāne śarīre (BG 2.20). It is eternal, śāśvata, existing forever. Na hanyate hanyamāne śarīre. By annihilation of this body, the soul does not die.