Text 27]
The Birth of Lord Kṛṣṇa
241
PURPORT
There are different categories of living entities, but everyone is afraid of death. The highest aim of the  karmīs  is to be promoted to the higher, heavenly planets, where the duration of life is very long. As stated in  Bhagavad-gītā  (8.17),  sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ : one day of Brahmā equals 1,000  yugas , and each  yuga  consists of 4,300,000 years. Similarly, Brahmā has a night of 1,000 times 4,300,000 years. In this way, we may calculate Brahmā's month and year, but even Brahmā, who lives for millions and millions of years ( dvi-parārdha-kāla ), also must die. According to Vedic  śāstra , the inhabitants of the higher planetary systems live for 10,000 years, and just as Brahmā's day is calculated to equal 4,300,000,000 of our years, one day in the higher planetary systems equals six of our months.  Karmīs , therefore, try for promotion to the higher planetary systems, but this cannot free them from death. In this material world, everyone from Brahmā to the insignificant ant must die. Therefore this world is called  martya-loka . As Kṛṣṇa says in  Bhagavad-gītā  (8.16),  ābrahma-bhuvanāl lokāḥ punar āvartino 'rjuna : as long as one is within this material world, either on Brahmaloka or on any other  loka  within this universe, one must undergo the  kāla-cakra  of one life after another ( bhūtvā bhūtvā pralīyate ). But if one returns to the Supreme Personality of Godhead ( yad gatvā na nivartante ), one need not reenter the limits of time. Therefore, devotees who have taken shelter of the lotus feet of the Supreme Lord can sleep very peacefully with this assurance from the Supreme Personality of Godhead. As confirmed in  Bhagavad-gītā  (4.9),  tyaktvā deham punar janma naiti : after giving up the present body, a devotee who has understood Kṛṣṇa as He is need not return to this material world.
The constitutional position for the living entity is eternity ( na hanyate hanyamāne śarīre, nityaḥ śāśvato 'yam ). Every living entity is eternal. But because of having fallen into this material world, one wanders within the universe, continually changing from one body to another. Caitanya Mahāprabhu says:
brahmāṇḍa bhramite kona bhāgyavān jīva guru-kṛṣṇa prasāde pāya bhakti-latā-bīja (Cc. Madhya 19.151)