Srila Prabhupada wrote in Dharma, the Way of Transcendence, Chapter 15, Bhakti is a Science, not a Sentiment:
The process of bhakti-yoga is not a concoction or speculation. It is a science. As stated in the present verse, Srimad-bhagavatam 1.2.20,
“Thus, established in the mode of unalloyed goodness, the man whose mind has been enlivened by contact with devotional service to the Lord gains positive scientific knowledge of the Personality of Godhead in the stage of liberation from all material association.”
Vijnana means “science.” In mathematics, “Two plus two equals four” is always true. You cannot make it equal five according to your whims. No!
In this regard, it is recommended that you read the essay, on this Web-Site, entitled, “Is Bhakti a Science?”. This essay will convince you, without any doubt, that “Bhakti is a Science”.
In finding a guru, we cannot say it is a science if we used all mystical and or popularity criteria that the neophyte student has no ability to verify. “Science” means verifiable, visible, reasonable, and consistent. Not invisible, arbitrary, and left to one’s whim or guesswork; which is the tendency of those who define guru in the most superlative terms and then conclude that no one is qualified or really qualified to be an advance guru. This is the opinion of those who probably judge their own intimate knowledge of their own disqualifications, then from an objective assessment of others.
Actually, Prabhupada defined guru in several ways, indicating that there are many varieties of gurus, because as in all matters in devotional service, there are varieties and gradations.
In devotional service, we must work from a broad understanding rather than from a keyhole vision. We fear being swindled, which is healthy, but in life, it happens that people push away from something unacceptable or problematic, something irrational like the present ISKCON system of rubber-stamping gurus.