In the State of Denmark there was the odor of decay... - Roger Zelazny

"Then the one called Raltariki is really a demon?" asked Tak.

"Yes — and no," said Yama, "If by 'demon' you mean a malefic, supernatural creature, possessed of great powers, life span and the ability to temporarily assume virtually any shape — then the answer is no. This is the generally accepted definition, but it is untrue in one respect."

"Oh? And what may that be?"

"It is not a supernatural creature."

"But it is all those other things?"

"Yes."

"Then I fail to see what difference it makes whether it be supernatural or not — so long as it is malefic, possesses great powers and life span and has the ability to change its shape at will."

"Ah, but it makes a great deal of difference, you see. It is the difference between the unknown and the unknowable, between science and fantasy — it is a matter of essence. The four points of the compass be logic, knowledge, wisdom and the unknown. Some do bow in that final direction. Others advance upon it. To bow before the one is to lose sight of the three. I may submit to the unknown, but never to the unknowable."

Beyond the River of the Blessed, there we sat down, yea, we wept, when we remembered Avalon. Our swords were shattered in our hands and we hung our shields on the oak tree. The silver towers were fallen, into a sea of blood. How many miles to Avalon? None, I say, and all. The silver towers are fallen.
…waters,where the stars shone like bonfires at night and the green of day was always the green of spring. Youth, love, beauty-I knew them in Avalon. Proud steeds, bright metal, soft lips, dark ale. Honor…

When I started writing my first novel, ...And Call Me Conrad, they always say: "Write about what you know" and I said "Well, if I get a nice sort of combination SF and Fantasy with these resonances from Greek Mythology it might be pretty good. It would also give me a chance to start filling in my background on all those things I don't know much about but should if I want to be an SF writer."
So I sat down and made a list of everything I felt I should know more about. Astrophysics, oceanography, marine biology, genetics... Then when I'd finished the list I read one book in each of these areas. When I'd finished I went back and read a second book until I'd read ten books in each area. I thought that it wouldn't turn me into a terrific, fantastic expert but I'd at least have enough material there to know if I was saying something wrong. And I'd also know where to turn to get the information I want to make it right.
While I was doing this, to keep the words and cheques flowing I wrote books involving mythology. And once I started picking up things involving astrophysics I'd write stories that played with those sorts of things. So that's why I started out with mythology.