How is a great poem created? It is a mystery. It is like asking for an explanation of a square cut by Gary Sobers or a cover drive by Rohan Kanhai. It is the same mystery that surrounds fundamental scientific discoveries.
One answer is that it is simply genius doing what it must. Shelley expressed this view:
“A man cannot say it, I will compose poetry. The greatest poet, even, cannot say; for the mind in creation is a fading coal, which some invisible influence like an inconstant wind, awakens to transitory brightness; this power arises far within,
