Perfect knowledge comes through submissive hearing from an authorized spiritual master, never through faulty personal speculation.Listen — Srila Prabhupada Uvaca
Prabhupāda: jñāna-yajñena cāpy anye yajanto mām upāsate ekatvena pṛthaktvena bahudhā viśvato-mukham [Bg 9.15] [Others, who are engaged in the cultivation of knowledge, worship the Supreme Lord as the one without a second, diverse in many, and in the universal form.] Now Lord Kṛṣṇa, after describing about the mahātmās, or the great souls, who worship the Supreme Lord by this process of kīrtana... Satataṃ kīrtayanto mām [Bg 9.14]. Śravaṇaṃ kīrtanam [SB 7.5.23]. [Prahlāda Mahārāja said: Hearing and chanting about the transcendental holy name, form, qualities, paraphernalia and pastimes of Lord Viṣṇu, remembering them, serving the lotus feet of the Lord, offering the Lord respectful worship with sixteen types of paraphernalia, offering prayers to the Lord, becoming His servant, considering the Lord one's best friend, and surrendering everything unto Him (in other words, serving Him with the body, mind and words)—these nine processes are accepted as pure devotional service. One who has dedicated his life to the service of Kṛṣṇa through these nine methods should be understood to be the most learned person, for he has acquired complete knowledge.] The process of devotion is nine different types. Śravaṇam. Śravaṇam means hearing. Kīrtanam.
Kīrtanam means chanting. Kīrtanam actually means describing. You can describe with music. You can describe with words.
You can describe in speeches. Any sort of describing, that is called kīrtana. And śravaṇam. Unless you hear, you cannot describe.
What you shall describe? If you do not know anything about the Supreme Lord, then how can you describe? Therefore hearing is the first item, śravaṇam. And the whole Vedic literature is called hearing, śruti, śruti-śāstra. Śruti means to receive hearing.
If you want to know the Supreme, you do not require to qualify yourself materially. You can remain what you are, but if you simply hear... God has given you the power of hearing. If you hear from authoritative sources, then you become perfect, simply by hearing.
Therefore the first principle, hearing, is recommended, śravaṇam. And formerly, the Vedas were heard by the students from the spiritual master. Just like in the Bhagavad-gītā you'll find that Arjuna is hearing from Kṛṣṇa. He's not studying any Vedānta philosophy in the battlefield. He was simply hearing.
So that is the process, hearing. You can hear at any place. Even in the warfield, you can hear from the authoritative source. So that was the process of acquiring knowledge, hearing.