Greatness is a transitory experience. 
It is never persistent.
It depends in part upon the myth-making imagination of humankind.
The human who experiences greatness must have a feeling for the myth they are in.
They must reflect what is projected upon them, and they must have a strong sense of the sardonic.
This is what uncouples them from belief in their own pretensions.
The sardonic is all that permits them to move within himself. 
Without this quality, even occasional greatness will destroy a human.