I heard recently from a Rabbi's speech (a.k.a., I don't know the actual source) that when the Alter Rebbe was in prison in Russia one of the Russians there asked him to prove the existence of G-d.
So the Alter first asked him to think of something that doesn't exist, the Russian said: a flying cow. The Alter Rebbe then said that that's incorrect, because a cow exists, and wings exist, he's just putting two things that already exist together, but it's impossible to actually think of / know about anything that doesn't exist, so the Alter Rebbe concluded that since we know about G-d, he must exist, because we know about him, and it's impossible to know about something that doesn't exist!
When I first heard this it sounded pretty good, but then I thought: why can't you just respond that he know of human creators of art (for example) and the "know" about G-d is just to consider everything as like a big art piece, and then to imagine some bigger creator, so it would just be like imagining something that does already exist, a human creator, and putting it in another perspective.
So I'm not trying to challenge the existence of G-d, chas v'shalom, nor am I challenging the wisdom of the Alter Rebbe, but I was trying to understand this particular proof from the Alter Rebbe (and the Rabbi I heard it from said it was found later in the Rishonim). So if anyone either #a can explain why it can't be refuted from the above or #b just find the source of this story somewhere ('cause it might clarify things to see it inside), then that would be the #1 answer.
EDIT just to clarify, the context in which this story was said was to explain the following: There's some parts of G-d we can actually know about, like how He creates the world(s) etc., like it's actually possible to know about / imagine the energy that gives life to every being, that's something we can comprehend, know about, imagine, etc. so relative to that level of G-d, it's considered an "existence", hence the Alter Rebbe's proof that G-d "exists", since it's referring to that aspect of G-d, which another aspect of G-d (which isn't at all related to the Alter Rebbe's proof) is that aspect of G-d that's totally removed from the world(s), it doesn't create them at all, it has nothing to do with actively giving life to the world, and regarding that level of G-d, it's actually not at the level of "existence", because we can't know about it, and (the Rabbi I heard the speech from goes on to say) these two levels correspond to "Memale" and "Sovev" as discussed in Kabbalah etc.
but anyway, the Alter Rebbe was just trying to answer the question of a Russian "How do we know there's a G-d at all!" and to that question the Alter Rebbe just gave the proof simply about since we know about it, it must exist (and he was only / simply referring to the first level "Memale", since he didn't want to get into deep Kabbalah with this Russian, just to simply explain that there is a G-d at all).
My question was only just why can't you say, according to this simple answer, that by "knowing about Him" the person is just putting other things that exist together to form a "Super Creator".
As I was just writing this, I realized I actually answered my own question that the proof G-d exists is because we know about the G-dly energy behind the worlds, as said above ,or even if someone hasn't reached that stage, he at least knows about the basic energy that gives life to everything, it's simple that everything (or at least) somethings have energy at least giving them life, and because we know about that, that's proof that it exists. So basically, I (with Double AA's help) actually answered my own question.