Prabhupāda: Just like who is a guru? That is stated in the śāstras. Śrotriyaṁ brahma-niṣṭham, śāstra says. Tad-vijñānārthaṁ sa gurum evābhigacchet (MU 1.2.12): "One must approach a guru." Then the same question comes, "Who is guru?" That is also stated, śrotriyaṁ brahma-niṣṭham: "He's well versed in Vedas, knowledge of Vedas, and fully Kṛṣṇa conscious." He is guru. Just like how do you know that, "Here is a medical practitioner"? Before going for treatment, you find out. How do you find out?
Śyāmasundara: Some friend, perhaps.
Prabhupāda: Anyway, you have to take information. Similarly, guru, according to śāstra, who is guru, he must be confirmed by sādhu, saintly person, by śāstra. Then he's guru. Sadhu-śāstra, guru-vākya, tinete kariyā aikya.
Haṁsadūta: What's the difference between a sādhu and a guru? Sadhu means authorities like Vyāsa?
Prabhupāda: Guru . . . he must be a sādhu.
Haṁsadūta: It means the previous ācāryas.