If you haven't defined God, how do you know it's not just "natural"? Natural means "not made by man". Don't you have to define what something is before rejecting it?
I'm interested in where you are going with this line of reasoning. Most contemporary atheists that are able to articulate themselves on the epistemological points will eventually get to the position: "Positive claims of the existence of X must be made with empirical evidence. Proving a negative existence claim is not, on principle, a scientific modality for knowledge."
Of course, it all depends on what it is you're trying to prove and with what evidence. The above applies for inductive methods, but you might think that you could argue a negative existence claim on the basis of the impossibility for something existing, and perhaps you could do that, say, mathematically.
With most atheists it doesn't seem to be the case that they're appealing to the latter. I'd put my own pocket money on the fact that most atheists would say they don't believe a deductive proof of the impossibility that God is T is even feasible.
Most of them want to hang their hats on a common sensical sort of empirical proof, and they don't believe that it's their burden to make the negative case that God is impossible; rather, it's on you to show that He is real, and not just probable.
I'm interested in where your thread (which I quoted) is going. If you took a metaphysical approach to cause a person to doubt that God is a substance that is amenable to empirical discovery, then you might be getting somewhere. I mean, to show that if what we think God is, is true, then God cannot be the sort of thing that is discoverable in just the way this the typical atheist is demanding that we prove Him.
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