Hayagrīva: God intervenes in the world through the individual acting according to scripture.
Prabhupāda: Yes. These is a word, sādhu śāstra guru vākya cittete koriyā aikya. We can approach God by understanding a saintly person, by studying the Vedic scriptures, and explained directly by the bona fide spiritual master. So sādhu means saintly person, and śāstra means scriptures, and guru means spiritual master—and that they should be corroborated. A sādhu is he who talks in terms of scripture. Similarly, guru is he who talks in terms of scripture. Guru cannot manufacture words which is not in the scriptures. And that is not scripture which does not tally with the words of guru and sādhu. So these three items should be corroborated, and then we can understand who is guru, who is sādhu, and who is, what is scripture. Then we take instruction from them, and we can perfectly make progress towards understanding of the Supreme Personality of Godhead.