The Sage tells us that disciples are of four grades, comparable to gun-powder, dry charcoal, ordinary fuel, and wet fuel.

The first kind of disciple needs only a word, like a spark, to consume his ignorance at once.
The second kind needs some teaching and personal effort.
The third kind needs a long course of teaching, training and practice.
The fourth kind needs to be made fit for discipleship by practices suitable to his condition.

— Maha Yoga, Chapter 11