Anything material that has got a birth, a stay for some time, a by-product, a growth, a dwindling, and then vanish—that is the nature, anything. Just like this body—it was born from the mother's womb at a certain time, and it is staying for some time.
It is staying for some time, and the body has got some by-product, like children. We have got some children, the by-product. Then it is dwindling. Just like I am getting older. Anyone, everyone, we are getting older. And at the last, it will vanish. Similarly, the whole material world, it has a time of its appearance, it grows, it makes so many varieties of by-product, it dwindles and again vanishes.
But Bhagavad-gītā gives you an information, paras tasmāt tu bhāvo 'nyo 'vyakto 'vyaktāt sanātanaḥ (BG 8.20). Beyond this material world, which is subjected to these rules of six changes, there is another world, which is sanātana. Sanātana means which is eternal. So actually there is existence of an eternal nature, like this nature which you are experiencing. And that nature, transcendental nature . . . the whole Bhagavad-gītā scheme is to take you back to that transcendental nature. Because you are transcendental, you are eternal, you are blissful, you are full of knowledge. Now we are covered. Now you have to go back to that eternal world, which is full of knowledge, full of bliss. So we have to prepare in that way. That is the policy of the human life.