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.
Prabhupāda: A sādhu means titikṣavaḥ kāruṇikāḥ suhṛdaḥ sarva-dehinām, ajāta-śatravaḥ śāntāḥ sādhavaḥ sādhu-bhūṣaṇāḥ (SB 3.25.21). Everything, there is definition, who is sādhu, who is guru.