(Krishna instructing Arjuna on the Battlefield of Kuruksetra)
"It is possible to become immortal, to have eternal blissful life of knowledge. That is called immortality. Sac-cid-ānanda. Sat means eternity and cit means knowledge and ānanda means pleasure, bliss. We are part and parcel of Kṛṣṇa. Kṛṣṇa is sac-cid-ānanda vigraha (Bs. 5.1). His transcendental body is eternal, blissful and complete knowledge, His body. Therefore He's speaking Bhagavad-gītā. If He's an ordinary man, what is the use of hearing Him? Ordinary man will commit mistake, will cheat, will be illusioned. His senses are imperfect; how he can give complete knowledge? That is not possible. Therefore we are not concerned to hear any rascal. We want to hear Kṛṣṇa. We are not prepared to hear any rascal, so-called scientists and so-called philosopher, so-called God. No. We are not prepared. Because everyone is rascal. Everyone is full of mistakes, everyone is trying to cheat others, everyone is illusioned, and everyone's senses are imperfect. How he can give knowledge perfect? That is not possible.
Therefore, we have to select guru. Guru means Kṛṣṇa. As Arjuna has selected guru, śiṣyas te 'haṁ śādhi māṁ tvāṁ prapannam: (BG2.7) 'I am, I become Your śiṣya, disciple, and I surrender unto You.' So to get perfect knowledge, we have to find out Kṛṣṇa or Kṛṣṇa's bona fide representative. Then there is perfect knowledge. Then we can become immortal. Unless we get perfect knowledge. So Kṛṣṇa is sac-cid-ānanda-vigrahaḥ, immortal. Kṛṣṇa is full of bliss, full of knowledge, and we are also part and parcel of Kṛṣṇa. So we have also the same quality, sac-cid-ānanda, the spiritual body. But because we have contacted this material nature, our blissfulness, our eternity, our knowledge, everything is now disturbed. Everything is now disturbed. We cannot be completely blissful. Anything you take, any pleasurable thing you take, it cannot give you always pleasure. It is not possible. This is not possible in this material world. However you may try scientifically to prolong your duration of life, live forever, it is not possible. And however you may pose yourself, cheat others, that you are the best man in knowledge, philosopher, scientist, you are a fool. That is not possible. Sac-cid-ānanda vigrahaḥ is Kṛṣṇa only (Bs. 5.1). So we can also become like Kṛṣṇa along with Kṛṣṇa. Not outside. We can be living force along with Kṛṣṇa. So Kṛṣṇa is also very much anxious to take us back to Him. Therefore He comes. Yadā yadā hi dharmasya glānir bhavati bhārata abhyutthānaṁ sṛjāmy aham (BG 4.7). When these rascals forget that unless he goes back to home, goes back to Kṛṣṇa he'll never be happy, that is the discrepancy of occupational duty. They have come here, kṛṣṇa-bahirmukha hañā bhoga vāñchā kare (Prema-vivarta), he has come here to enjoy, to lord it over the material nature, and has become entangled. So Kṛṣṇa comes, descends, to save us, to give us shelter at His lotus feet so that we may also become immortal like Kṛṣṇa, full of bliss and full of knowledge. That is called amṛtatva.
So unless we have got our aim, target of life, then what is the value of life? This is amṛtatvāya. Is there any institution, is there educational institution or university or college where this teaching is given, that how you can become immortal? Is there any institution in the world? Throughout the whole world? No. They are simply teaching that you live like animal and die like animal. That's all."
(Srila Prabhupada Lecture, London, August 21, 1973)