The following is my last favorite kind of argument in philosophy because it is so divorced from reality. This kind of argumentation has been endlessly scrutinized and rightly so. But let's play around with it anyway. Here's Anselm's ontological argument. It goes a little something like this.
(0) God is a perfect being.
(1) It's possible for us to think about God.
(2) What we can think about exists in our minds. (?)
(3) So God exists in our minds.
(4) God exists only in our minds or he exists in our minds plus in reality.
(5) If something exists only in our minds, then it's not perfect. (?)
(6) So if God exists only in our minds, he's not perfect.
(7) But God is perfect.
(8) So God doesn't exist only in our minds.
(9) So God must exists in our minds plus in reality.
This argument can be attacked on two grounds. Peep the question marks next to the above propositions.
The first attack can be waged against the second premise. Saying that something "exists in our minds" is an interesting piece of sophistry. When I think about a piece of cow entrail, does it really exist in our mind? An odd locution! Toss it to the flames.
The second attack can be waged against proposition five. Suppose we forget about whether or not something "exists in our minds." What does it mean to say that the thing that exists only in our minds is not perfect? This assumes "existence" is something like a property one can possess. But huh? I don't know what it would mean to say that something is not perfect because it doesn't exist. I might wish for many things to exist that don't exist but to call a nonexistent something-or-other imperfect on those grounds is just to play with words.
I wish I had some lasagna.