There are actually quite a few problems with this "argument" however. The first is the assumption that there are only two positions to take on the God existence question. That the question of God's existence is either true or false. There is actually a third, that of the unknown. It is possible for a person to not know either way if God exists or not. We call these people agnostics.
The atheist would object that for all functional purposes an agnostic and an atheist are interchangeable. This is true at the functional level. But it is not true at the proposition level. An agnostic does not say that God does not exist. An agnostic states that he doesn't know if God exists or not. An atheist asserts that God does not exist. An atheist is not netural on the question.
A theist points out that the atheist has no standing either on the question. But at this point the atheist employs a sleight of hand. The statement is that since it is impossible to prove a negative (i.e. God does not exist) it is only proper to assume that God does not exist and prove that He does. This is absurd for two reasons.
The first is that as pointed out above we are not locked in by two positions. The "default" is the stance that we cannot conclude one way or another. Thus the assumption should be that we dont' know if God exists or not.
The second problem is that if it is impossible from the evidentiary standpoint to prove a negative, then there can be no such thing as a rational atheist. It is not possible to prove the non-existence of God by the rules set forth by the atheist. The only thing one can conclude in the negative is that we do not know if God exists or not. There is no way to assert that God does not exist if evidentiary arguments are what is required.
What the atheist in this case is claiming is "I cannot prove God does not exist. Thus I will simply assume I'm right and force the theist to do the heavy intellectual lifting." It is ultimately an argument in bad faith. The atheist assumes something he cannot possible know by his own framework. It is an intellectually lazy position to take, and one that reason cannot argue against because if taken on its own terms reason was not how it was arrived at.
The theist should rightly point out that this position is absurd and force the atheist to concede or argue against three things:
- That under this framework there is no way to prove God does not exist.
- That the atheist position as the default has nothing to support it.
- That evidentiary arguments are not necessary (indeed that they are not even appropriate) for determining the existence of God.
