If a student (or students) voluntarily and enthusiastically come to learn from an individual, that individual is a great teacher.

The emphasis is on “to learn” because some people confuse great charisma with great pedagogy, but such people go to the charismatic person to take comfort from being in that person’s presence, not to learn — despite whatever truisms or clever-sounding parallel grammatical structures the charismatic individual might spout.

So, if one would go to an individual seeking knowledge, knowing the experience will be challenging and not merely comforting, one has found a great teacher.