Guest (1): How can we know that somebody is perfect?
Prabhupāda: That is another thing. But first of all, the basic principle is we have to understand that our senses are imperfect, and whatever knowledge we gather by this imperfect senses, they are imperfect. So if we want perfect knowledge, then we have to approach somebody who is, whose senses are perfect, whose knowledge is perfect. That is the principle. That is the Vedic principle. Therefore the Vedic principle says, tad-vijñānārthaṁ sa gurum evābhigacchet (MU 1.2.12). You know Sanskrit, yes? "In order to know that perfect knowledge, one should approach guru." So who is guru? Then the next question will be. Your question is that, "How I can?"
Guest (1): How can I know that . . .?
Prabhupāda: That I am coming. That I am coming. Guru . . . that is next line. It is said, śrotriyaṁ brahma-niṣṭham. Guru means who has properly heard the Vedas, śruti. Śrotriyam. And as a result of his hearing he is firmly convinced in the existence of the Absolute Truth, God.
Professor: Well, this is . . . we've only come to one of the, as I mentioned, the theories of knowledge, I think, śabda.
Prabhupāda: Sata?
Professor: Śabda.
Prabhupāda: Śabda, yes, śabda-brahman. Yes.
Professor: Then if you are able to communicate to have this knowledge through śabda, what?
Prabhupāda: Yes. Śabda-brahman. Just like many thousands of miles away we are getting some radio message and we learn that, "Something is happening there. Something is there." Therefore śabda. This is . . . śabda means sound. Sound. Sound vibration. So that is the real source of knowledge. That is the real source of . . . śabda-brahman.
Professor: One of the sources of knowledge, of the only one?
Prabhupāda: No, that is the only one. There are others; they are subordinate. But the śabda, knowledge received, śabda, through śabda, śabda-brahman, that is perfect knowledge. Just like the same example—beyond this wall I cannot see, but if somebody there says: "This is the position here"—the sound comes—that is perfect. You cannot see what is going on, but if somebody says . . . sends radio message or any message, sound, then you know. Therefore śabda-pramāṇa, śabda, knowledge received through śabda, that is perfect knowledge.