In the comments section of my review of Stenger's book, a person calling himself ReallyEvilCanine raised an interesting question:
The common aphorism "Absence of evidence is [...?] evidence of absence" is the same fallacious logic used by those who believe in some deity. There's no difference between the following two versions of the Appeal to Ignorance:
* There's no evidence to disprove X, therefore X exists.
* There's no evidence to prove X, therefore X doesn't exist.
I've had this discussion before, and it can be complex. Is there evidence for Christianity or not? There is evidence. But what is it evidence of? Stenger claims science can test the evidence, and when it does, there is a lack of evidence for Christianity. Christians dispute this, of course, but Stenger makes a good case, nonetheless.
Most all of the evidence on behalf of Christianity is that science cannot explain everything, i.e., "if there's no evidence to disprove X, therefore X exists." If science cannot fully explain consciousness, the origins of morality, logic, the laws of science, and the origin of this universe itself, Christians take this as evidence that their faith is true. This is called the God of the gaps defense. God is to be found in the gaps of our knowledge.
There are twin problems for this kind of defense. One problem, as Martin Gardner explains, is that there will always be gaps in our understandings. Therefore, there will always be room for the theist to believe. The other major problem is that the believer is demanding an unreasonable standard before the evidence can actually show his faith to be wrong. The believer is demanding that the evidence must eliminate all possibility that what the believer claims is true. That's an impossible evidential standard, as I've indicated. In no other area of belief do we demand this impossible standard.
What the believer should admit is that at best, "if there's no evidence to disprove X, therefore it's possible that X exists." But you see, since there are a great many things that are possible but not actual, such a conclusion doesn't gain the believer much ground at all.
Furthermore, the scientist does not claim "if there's no evidence to prove X, therefore X doesn't exist." What he actually says is this: "if there's no evidence to prove X, therefore X probably doesn't exist." Does this appeal to ignorance in the same way as the believer does? I think not. Consider how science confirms theories. Science confirms theories based upon logical fallacies. Consider this scientific argument:
If scientific hypothesis P is true, then experiment Q will obtain.
Experiment Q obtains,
Therefore scientific hypothesis P is true.
The form of this argument is invalid. It looks like this:
This is a fallacy called affirming the consequent, and yet that's how science proceeds for the most part when it comes to confirming hypotheses. Science doesn't prove hypotheses, even if the experiment obtains, because the hypothesis could still be false even if it does. Perhaps the experiment didn't actually test the hypothesis accurately? Perhaps premise one is a non-sequitur?
If this is true of science in general, then it becomes even more problematic when science investigates metaphysical beliefs. When it comes to these beliefs there comes a point when one person's fallacy is another person's anomally.
Think of it this way. If the claim is that the Russian government massacred 800 people in a farmland near Moscow in 1968, and there is no physical evidence that they did, what should we conclude? At that point all we have is the lone survivor's claim. Whether he is believable depends on whether we believe the tale he tells and what we think the Russian government might have done to the people he claims were killed.
What about the claim that Santa Claus or an Easter Bunny exists? What about the claim that Zeus or Apollo exists? There is no scientific evidence for such beliefs that I know of. None. So we must examine those claims pretty much the same way we would do with the claim that the Russian government killed 800 people in 1968, with an exception. We would have an additional problem with believing in so-called supernatural beings, since we have seen them come and go depending upon the culture of those who believe.
As best as I can determine it, the phrase, "absence of evidence is evidence of absence," merely describes how science operates. Science looks for evidence to support a theory. If no evidence is found for the theory, then this lack of evidence is taken to be reason to think the theory is not probable. That's all science can say, and it does it's job well.