1) Do you think Lewis believed in their existence, since he put them in his book?

Maybe. He might have had suspicions one way or the other. He might have just thought it was a good origin for Jadis, to show her true character.

2) Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?

Oh wow, Lilith fits Jadis perfectly! What Lilith does is similar to what Jadis does in LWW: lure Edmund to go astray.

3) Do you know anything additional about this subject?

I watched the History Channel show by the Naked Archeologist on this subject, and was amazed by how much archeology there was about Lilith. She was said to be of the same earth that Adam was made from, and later Eve was made of Adam's rib. I don't know if this is true or not (the Lilith part), but it is interesting to discuss.

Oh wow, Lilith fits Jadis perfectly! What Lilith does is similar to what Jadis does in LWW: lure Edmund to go astray..

Yes, I agree. Edmund is both a child AND a boy, thus doubly influenced by Jadis.

1) Do you think Lewis believed in their existence, since he put them in his book?

No, he was a mythology guru and just wanted to incorporate some other mythologies (or allusions to others) into his books. Maybe he wanted children to research them for themselves and make up their own mind about them.

2) Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?

Probably because Lilith is a distortion of God's Truth (Jewish mysticism is very different to the Jewish scripture), or because they're mysterious and eerie and other-wordly. If you know of the creatures and their origins, you know Lewis is hinting at a demonic-type creature that can take the guise of humans and is not to be trusted (cunning, crafty etc.)

Maybe he wanted children to research them for themselves and make up their own mind about them.

Hmm, well if so he certainly suceeded!

Probably because Lilith is a distortion of God's Truth. If you know of the creatures and their origins, you know Lewis is hinting at a demonic-type creature that can take the guise of humans and is not to be trusted

Ah, very good point. I agree perfectly. I think Lewis really gave Jadis more realism and depth by adding that.

Since I am a Christian, not Islamic or Jewish, I do not believe in the existence of Lilith or Jinn...

That's a common attitude. I've always found it a bit amusing. But seriously, we all pick and choose what legends to believe in. Most believe in the resurrection, some also believe in the flood, and the nephililm, and in any number of other crazy things.

One example: I'm fairly certain that at least one old testament writer knew about celestial mechanics and gravitation. In Job, we read "you have suspended the earth midst the void upon nothing!"

Remember what Bilbo Baggins said about dragons: "they're far away, and therefore legendary." In fact, one of Lewis favorite and recurring themes is when one or another character discovers that some otherwise crazy legend... turns out to be true. Those with receptive minds shrug and deal with it; those like Eustace when he sees his talking mouse, freak out and go to pieces.

...but i'm not going to out-rule all possibility

Exactly so, until your experience or careful research turns up useable facts, lack of evidence proves nothing. The west is far too dismissive of things that do not fit into its present narrow understanding of reality in part because we're used to having a science that "knows the answer." We've become like the sadducees. Modern science is a joke, we're too good a deceiving ourselves. We're uncomfortable with situations where "Nobody Really Knows."

that's my answer to question #1.
I agree with some of the above comments about question #2.
I respectfully assert my fifth amendment rights on #3.

"Nobody Really Knows." is actually the thing that I tend to stick with on contreversial things that I do not understand. I find mystical crazy things interesting, but untill I have studied them and heard multiple points of view I make no definite conclusions.

1) Do you think Lewis believed in their existence, since he put them in his book?

One might as well have asked if Lewis actually believed in Talking Trees. At some point, allegory stops, and fantasy kicks in. He chose to set Narnia as a magical (and therefore unrealistic, in some regards) world. Not everything he wrote in the Chronicles has to be accepted a Gospel truth.

2) Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?

Keeping in mind that Lilith is a Jewish legend, and not based in Scripture, both the characteristics given her and those ascribed to genii fit Jadis' people. They were first wise and happy, given powerful magic to rule their kingdom. But corruption and pride became their downfall. In the end, the last of their line used magic to destroy everything but herself - surely the most selfish and evil of all acts in the books.

3) Do you know anything additional about this subject?

About genii? Only what I've read in Arabian Nights. About Lilith? That many people, including the ancient Jews, had stories mothers invented to keep their kids in bed at night.

One might as well asked if Lewis actually believed in Talking Trees.

Haha you are right of course. That makes me feel silly

1) Do you think Lewis believed in their existence, since he put them in his book?
Did he believe in water gods, talking beast (ok maybe I believed that one), or Dragons? Certainly not ones in our world
2) Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?
I think it was chosen more to fit the character than anything else.

With Lilith you get:
Human appearce/beauty
Dislike of humans
Need for power
Mortality

With the Jinn you get:
Magical powers
Height (I think they were rather tall but I'm not sure)
etc.

3) Do you know anything additional about this subject?
Other than that genies have "Ultimate Cosmic Powers!! And a little tiney tiny living space...

Questions for discussion:
1) Do you think Lewis believed in their existence, since he put them in his book?
No. He put a lot of mythological characters in his book. They didn't all fit together even as mythology, and I don't think he believed any of them to exist for real.
That was not his point - he was writing a fairy-tale, after all!

2) Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?
He chose Lilith because she was connected to Adam in the myths, and he needed some connection to Adam for her to enable her to pretend that she had the right to be Queen of Narnia according to the "Son of Adam, Daughter of Eve" rule. I don't know why he chose Jinn - probably to give her some characteristics that he wanted her to have.

The strange thing is that by the time of MN he seems to have forgotten all about this. There's no reason Adam or Lilith should have any connections to Charn, and Lewis doesn't try to give us any such reasons.
With the Jinns it might make more sense, but he doesn't mention it - he talks about giants instead.

"Hardly human" was what Digory thought when he looked at her; and he may have been right, for some say there is giantish blood in the royal family of Charn.

3) Do you know anything additional about this subject?
Nothing that I can think of right now

The way I've always seen it is that the description of Jadis' origins in MN is the actual truth, and the description of her origins in LWW is what Mr. Beaver THINKS is the truth, but is inaccurate. Mr. Beaver strikes me as one of those rather pompous individuals who's fond of coming out with explanations for everything to show how clever they are, when they don't really know much about the subject at all. He tells the children that no humans have been to Narnia before, when the story of Digory and Polly would most likely still have been common knowledge among the Narnians.

The way I've always seen it is that the description of Jadis' origins in MN is the actual truth, and the description of her origins in LWW is what Mr. Beaver THINKS is the truth, but is inaccurate.

While 'Lewis forgetting' is a possible (but unprovable) Outside-of-the-story explanation, this would be a quite good Inside-the-story explanation

It doesn't explain what is said during the conversation between the Witch and Aslan when they negotiate about Edmund, though. Things are said there about Jadis' role in Narnia that are not compatible with the story in MN about how she enters that world. (They would not be dependent on her racial origins, though.)

Mr. Beaver strikes me as one of those rather pompous individuals who's fond of coming out with explanations for everything to show how clever they are, when they don't really know much about the subject at all. He tells the children that no humans have been to Narnia before, when the story of Digory and Polly would most likely still have been common knowledge among the Narnians.

Not to mention King Frank and Queen Helen - their line would have died out in Narnia (possibly not in Archenland), but it should be part of known history to know that they were humans.

1. Do you think Lewis believed in their existence, since he put them in his book?
I agree heartily with Varnafinde. Lewis included Satyrs, Fauns, Monopods, Bacchus, Merpeople, centaurs, Giants, and the like--did he believe that these were myths? Yes. Whether or not he believed that the idea behind said myth is something we can't know for certain--but I'd say that it'd be fair to say he didn't believe in them. He might have thought of them as demons or ideas created by demonic suggestion, but that'd be pure and total conjecture.

2. Why do you think C. S. Lewis chose Lilith and Jinn to be the origins of Jadis?
Story detail, mainly, and secondly because Lewis really enjoyed Myth. It suits both Jadis' personality and her appearance, and thus gives the reader an idea of what they're getting into, if they happen to be familiar with those legends. If not, then they have something to look up later. Also, I somewhat agree King_Erlian's head-canon. It's been my thought that the Witch's lineage has been rather twisted, misunderstood, and generally muddled over time (according to the time line, she'd been in Narnia for over 500 years before making a move--think of it as a massive Telephone Game, except even the original line wasn't fully explained to start with).

Lewis used a lot of mythology in his books, including minotaurs, centaurs, mermaids, etc. but I honestly can say I don't believe he thought any of them actually existed. It's the same here - Lewis was adding mythology to his books from various sources, including Greek, Irish, Jewish, Arabic and other kinds of mythology, not because he actually thought they were true, but because they added an element of mystery into the book. I think he used mythology here to explain why the witch looked so much like a human, but in reality was not, and not because he was giving reference to where witches really come from (mixed Jewish and Arabic mythology). Lewis' point on witches' existence is given very strongly in "Mere Christianity" -

One man said to me, "Three hundred years ago people in England were putting witches to death. Was that what you call the Rule of Human Nature or Right Conduct?" But surely the reason we do not execute witches is that we do not believe there are such things. (emphasis added)."

-(Mere Christianity, Chapter 2: Some Objections)