Yet another question on 「は」 and 「が」

Question

Below are two excerpts from the short novel 「タクシーに乗った吸血鬼」 by Murakami Haruki:

その一

「でもね、本当に吸血鬼がいたらどうします？」

「参っちゃうだろうね」

「それだけですか？」

「いけないかな？」

「いけないですよ。信念というのはもっと崇高なもんです。山があると思えば山がある、山がないと思えば山はない」

その二

「お客さん、信じてませんね？」

「ん？」

「私が吸血鬼だって……信じてないでしょう？」

「もちろん信じてるよ」とあわてて僕は言った。「山があると思えば、山はある」

The questions:

1. I read the post on the topic marker used in negative statements (as a contrast marker), but I can not see what the mountain in excerpt 1 is contrasted with. Is it contrasted with the mountain that turns into being when you believe there is a mountain? Or is it contrasted with anything else that does exist? Is it really contrasted with anything at all?
2. I have no idea why 「は」, instead of 「が」, is used in the second occurence of 「山があると思えば山（は/が）ある」 in excerpt 2.

Answer 1

I would say that in the first instance the ideas being contrasted are the effect of belief: ie., whether you think there's a mountain or not. So, the idea expressed by 山はない is contrasting against idea in 山がある.

Normally what we think about things (like mountains or vampires) does not effect whether or not there is indeed a mountain (or vampire). If I were to try to capture this in English, I would write it something like,

If you think there's a mountain, then there's a mountain. If you think there isn't a mountain, then there's no mountain.

In the second instance, I would say this is an instance of emphasizing the mountain. It would be almost like saying,

If you think there's a mountain, then indeed there's a mountain.

Granted, there's no word in the Japanese that corresponds to the word, "indeed". But the feeling created by は here (particularly because it says もちろん信じてるよ」とあわてて僕は言った) is that of emphasizing the consequence of the belief.

Since I haven't read the story, my guess about what's being said is that if you believe there's a vampire there, well, in effect, there is (because you're going to act accordingly). If you don't believe there's a vampire there, then there isn't (again in terms of how you act).

Answer 2

As for why it says 山はない in the first example

Since は is attached to 山, instead of 山がありは（しない）, it doesn't stand for contrast between being and not being. In addition, the prior information is the same 山. So, it's not really reasonable to think that it contrasts with something. In this regard, you could explain that it sets a new secondary topic under the primary 信念というもの. But it's more straightforward to say that it simply agrees with the negative predicate. This is kind of a conventional usage and doesn't have particular reasons. You could even say it's because it's familiar with your mouth.

Edit: There's a room to interpret that the second 山 is recognized as an old information that corresponds with the first 山, that is, "If you believe, you get a mountain. If you don't, the mountain goes away".

As for why it says 山はある in the second example

As you say, 山がある would be just natural here because it's a consequence after the conditional clause, which functions as a topic in a sentence. You don't have to topicalize it. However, if you nevertheless do it, it conveys an effect to make listeners wonder what will happen to 山 then when it's presented as a new topic, and bring the consequence a bit delayed, which is so to speak a nuance of "after all" or "indeed" as A.Ellett says.

Answer 3

「山があると思えば山がある、山がないと思えば山はない」

Is it contrasted with the mountain that turns into being when you believe there is a mountain? Or is it contrasted with anything else that does exist? Is it really contrasted with anything at all?

「山があると思えば、山はある」

I have no idea why 「は」, instead of 「が」, is used in the second occurence of 「山があると思えば山（は/が）ある」 in excerpt 2.

In your examples, が can be replaced with は no problem. Just it will sound more generic and lose the realistic feeling が can create, but it will get a broad feel to it instead.

However, if you replace these は of each sentence with が, it will assume quite a different tone; it will sound very much like the mountain has actually disappeared or appeared!!

Do you see, in each sentence, は is the one that is indicating the subject of the whole sentence while が is marking the subject in the modifying clause?

Both は and が have the role of indicating the theme/topic/subject of the predicate, but in Japanese, as you might already know, they are not necessarily the action maker. The difference between them is that は is good for a general idea while が is good at introducing a happening.

There's several subject markers (係助詞｛かかりじょし｝) in Japanese, but for us native speakers, は is the most prominent of them. On the other hand, が is defined as the case marker (格助詞｛かくじょし｝). So to speak, は is bigger than が.

As user4092 says, が introduces the second theme because it's good at working in a subordinate clause. I can't generalize anything here, but I think we can, most of the time, add something with は such as 今回は, or 今日は or そこには etc. when we have a sentence which starts with XXが.