Asmin dehe, "In this body, there is the proprietor, the soul." Dehino 'smin yathā dehe. That, on account of the proprietor, he is changing body. Changing body means . . . so long the soul is there. Suppose a child is born. If the child is born dead, then this body will never grow. You can apply any chemicals or any science; the body will remain the same. But so long the soul is there within the body, the child from the babyhood will come to childhood, then childhood to boyhood, boyhood to youthhood. In this way the body will change. We have changed so many bodies, every one of us. I knew . . . I know that I had a childish body, I had a boyhood body, but those bodies are no more existing. But I am existing. Therefore the conclusion should be that I, you, as soul, we are eternal. The body is changing.
This is our disease. Therefore this disease . . . this disease means birth, death, old age and disease. So as soon as you accept this body, material body, you become subjected to the four laws of material nature. These four laws of material nature are that as soon as you've accepted this body, then you must accept death. Anything which is born must meet death also. Birth, death. And in the via media there is old age and disease. This body, I have got, you have got, everyone. There is a death of getting this body, and there will be a death of leaving this body. And between these two deaths there are so many other miserable conditions. They are summarized: old age and disease.
But the real science is that "I am the soul. I am the part and parcel of the Supreme Lord. So Supreme Lord, God, is eternal. I am also eternal." These things are described. Na hanyate hanyamāne śarīre (BG 2.20). As God . . . we are as good as God, at least in quality, not in quantity. We are . . . just like a drop of seawater is as good as the seawater in quality—the whole seawater is also salty, and the drop of seawater is also salty—similarly, we have got all the chemical composition, or qualities, of God. Now, God is eternal; therefore we must be eternal. That is stated in the Bhagavad-gītā.