What is an argument from ignorance?

Traditionally arguments from ignorance go something like this; there is no reason to suppose that P is false, therefore P must be true, or vice versa; whatever has not been proven true must be false. Arguments from ignorance are alleged to be used in many areas of natural theology. One classic example would be a common argument used by Christian theists that evolution can’t explain how life developed therefore God must have created life.

My problem is this, arguments from ignorance are simply easy to avoid. All the theist has to do to avoid using this argument is to change the nature of the claim and say that God is a better explanation of how life developed than evolution, thus we should believe God is the explanation over evolution. There certainly are other difficulties with this argument, but an argument from ignorance is not one of them.

“But…” the skeptic might retort, “how then do we explain why using aliens to explain U.F.O.’s is irrational?” A fair question to be sure. The answer may sound a bit odd at first, abandon the claim that there is no evidence that aliens exist. The fact is that aliens as an explanation does have explanatory power and as such the claim that aliens exist has evidence for it. However, the problem here is that there are multiple plausible explanations, U.F.O.’s could be explained by astroids, human developed experimental aircraft, or some unidentified natural phenomenon such as ball lightning. Because each of the explanations explain the same range of events and phenomena it is impossible, without further analysis, to argue that one explanation should be preferred over another.

The irony is that if we are to take the traditional view of “arguments from ignorance” which is so often used by skeptics and atheists, then we come to the conclusion that the atheist argument that because there is no evidence for God’s existence it is improbable that God exists commits this fallacy. As Carl Sagan noted, “absence of evidence is not evidence of absence.” However, I don’t think the argument commits this fallacy. This is because atheists who use this argument are really arguing that if God existed we would expect to see more evidence of His existence. Thus the argument in its most effective form can be put in a simple syllogism;

1. If God exists there must be  powerful physical or philosophical evidence of His existence.
2. There is no powerful physical or philosophical evidence of His existence.
3. Therefore God does not exist.