(Prabhupada's lecture - Śrīmad-Bhāgavatam 1.16.16 - Los Angeles, January 11, 1974)
And the memory, memory also reduced. Formerly, people were so sharp in memory that once heard from the spiritual master, he will remember. He will not forget. There was no need of books. Book is required because our memory is now, the power of recollection, is very poor. Just like one who cannot memorize, he has to take note down. Formerly, five thousand years ago, there was no need of books. The students will go to the spiritual master or teacher and... Therefore it is called śruti. The Vedic language is called śruti. Śruti means hearing. It is not reading. Simply, pious students will hear from the right source. Evaṁ paramparā prāptam imaṁ rājarṣayo viduḥ [Bg. 4.2].
Simply by hearing, they will understand everything. They will never forget. But in the Kali-yuga it is not possible. Therefore Vyāsadeva, the whole Vedic understanding, knowledge, he recorded into books. Otherwise before that there was no need of books. The power of recollection will diminish. And mercy. Mercy will reduce. Even in your, in somebody's, in our front, somebody is killed. Nobody will try to help him. What to speak of others, when there is difficulty. Or there are so many. People should cooperate.