It sort of sounds like St Anselm's ontological argument for the existence of God, basically an a priori argument which is difficult to disprove. However, the concept of God in this argument is in a sense rather vague, and usually the argument is limited to a specific aspect like "greatness" or "power". For me, the faulty assumption is that existence makes something "greater" or more powerful than if it existed in the imagination. But if you accept this assumption and define God as "that than which nothing greater can exist" then, for this unique concept, you have supposedly proven in your mind that God exists.
But that faulty assumption is a big one (existence as a great-making quality). Also, the definition of God in this argument includes almost nothing of the other characteristics usually attributed to him, not even a "him" (implying a person of some sorts).
I used to look at the ontological argument, especially as put forth by St Anselm, as brilliant theology/philosophy, when things like empirical evidence were less important to me. But all this argument really proves is that, if you truly understand the concept of God as defined in the argument, and if you agree that existence is a great-making quality, then God exists - in your head. Doesn't prove anything about actual existence in the world outside your mind.
It's a fun mind game, though. Similar games/arguments can be had regarding other ideal concepts (love, justice, perfect forms).
The problem - as I see it - with Anselm's argument is that it is flawed right from the start - there is an unstated (and faulty) premise that something can exist in the mind. The concept of God may exist in the mind but the concept of God is not God - just like a picture of an apple is not an apple.
Perhaps it's time to bring up Russell's Celestial Teapot argument?
If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time.
- Bertrand Russell
Now, I'm talking about a Celestial Teapot, therefore a Celestial Teapot is on my mind. Therefore I believe in the Celestial Teapot!
Of course, you might have to use a different example because the Celestial Teapot actually does exist. Now excuse me, I need silence as I go to perform the first of my Daily Brewings *lights candles*
There's probably nothing you can say that will penetrate this chap's magical anti-logic forcefield. In order to have a debate, both participants need to be capable of basic logic and reasoning. Anything else will just result in reason bouncing off an inpenetrable wall of stupidity.
You are obviously involved in these discussions out of your friend's belief in god, not out of any belief of your own. It is not god that is on your mind, it is the effects of other's beliefs. Your friend is either misunderstanding or deliberately misconstruing your intent. I don't know if there's a precise logical fallacy involved here, but I'm no real expert on the terminology.
Sojourner has a good explanation, but this is actually a form of the typical 'ontological argument', which theorizes that merely because you can conceive of God means that God must exist. These arguments are obviously absurd.
A) If you are thinking of something, then it exists.
B) If you're arguing for or against something, you're also thinking about it.
C) You're arguing against God's existence.
D) Therefore God exists (because you're thinking about him^)
That argument is fine in that, for that particular example, it's a valid argument (all points of the argument follow in line), but to test it out you have to apply it in other contexts to see if the proposal holds water. Consider these:
A) If you are thinking of something, then it exists.
B) If you're arguing for or against something, then you're thinking about it.
C) You're arguing against the existence of leprechauns
D) Therefore, leprechauns exist.
Technically these arguments 'follow' (sequitur) in their particular lines of reasoning and are superficially valid, but in logical arguments you must also examine the facts and deeper logic at hand, and regardless of whether or not the proposal matches the conclusion of the argument, we know a valid argument is not always a good one.
A) If you're thinking of something, then it must exist.
B) I'm thinking there's a miniature pink unicorn trotting around on my palm
C) Therefore, there must be a miniature pink unicorn trotting around on my palm
Obviously there isn't, so while valid, the argument isn't sound (an argument that is sound must have a true premise, as well as a valid argument). The premise here is not true, though, by essence, therefore you're allowed to toss the argument out the window, so to speak, without a second thought; it isn't worth arguing a premise that is provably untrue (you can run tests for yourself. . . I'm thinking of a million dollars showing up at the end of my bed in five seconds in a nice fat black suitcase. . . oops! Didn't happen! Guess it must not be true).
Good luck with that, though. Ontological arguments are an Atheist's worst enemy, because they're arguments that involve theists sticking their fingers in their ears screeching 'LA LA LA LA LA Can't hear you! LA LA LA~!'
Well for sake of argument, my post works even with 'you think of. . . therefore you believe. . .' -- If I'm thinking of a little pink unicorn prancing around in my palm, that doesn't mean I believe it's going to happen.