Ignorance, Incredulity, and God-of-the-Gaps

It is not uncommon in discussions over controversial topics for one side to accuse the other of fallaciously "arguing from ignorance" or from "personal incredulity". In discussions of intelligent design versus Darwinian naturalism the closely related "God-of-the-gaps" objection is frequently leveled. I believe that most attributions of these fallacies are uncharitable and undeserved.

Consider the argument from ignorance:

There is no evidence for X.
Therefore, X is false.

Clearly, this is invalid. If the only information we have bearing on X is that there is an absence of evidence for X, then the inference is clearly fallacious. But is it typically the case that that's the only relevant information available to the arguer? Let's change the argument slightly:

There is no evidence for X.
If X were true, then we would have evidence for X.
Therefore, X is false.

This is a perfectly reasonable argument strategy. It fact, it's an instance of modus tollens. Hence, whether one commits a fallacious argument from ignorance depends on whether or not one is justified in believing the appropriate if-then premise connecting the lack of evidence for X with the claim that X is false. And in many cases we are justified in believing that premise. For example, suppose that I have lost my keys and think they might be near my nightstand. I go to my nightstand and do a thorough search of the area and do not find the keys. There is no evidence that my keys are near my nightstand, but in the light of my search my keys really should have turned up by now if they had been near my nightstand. So I do not reason fallaciously if I conclude that my keys lie somewhere else. I suspect that many (if not most) allegedly fallacious arguments from ignorance are like this. They are really instances of modus tollens with a unstated conditional premise.

Now consider the argument from incredulity:

I cannot imagine how X could be true.
Therefore, X is false.

Again, this is clearly invalid. If the only information I have bearing on X is my own inability to imagine how it could be true, then the inference is clearly fallacious. But now consider the following:

I cannot imagine how X could be true.
If X were true, then I would be able to imagine how X could be true.
Therefore, X is false.

This revised argument is not fallacious, but a valid instance of modus tollens. As before, everything hinges on the conditional premise. If I take myself to be highly intelligent and informed about matters related to X, and if I've seriously tried to come up with a plausible argument for X, then I think I'd be in a pretty good position to affirm that premise. I suspect that many (if not most) allegedly fallacious arguments from incredulity are really instances of modus tollens with a unstated conditional premise.

Finally, the so-called "God-of-the-gaps" fallacy can be handled in exactly the same way.

X is a fact that stands in need of explanation.
There do not appear to be any adequate naturalistic explanations for X.
Therefore, there is a supernaturalistic explanation for X.

As it stands, this is problematic because the fact that there does not "appear" to be an adequate naturalistic explanation may be due to intellectual laziness or ignorance on my part. But if I've done my homework well - if I am sufficiently intelligent and informed, if I have identified decisive flaws with existing naturalistic explanations (if any), and if I have seriously tried and repeatedly failed to come up with a workable naturalistic explanation, then I can reason cogently as follows:

X is a fact that stands in need of explanation.
There do not appear to be any adequate naturalistic explanations for X.
If X had a naturalistic explanation, then we ought to have come up with an adequate one by now.
Therefore, there is a supernaturalistic explanation for X.

In conclusion, we ought to be very careful before accusing people of committing these types of fallacies. If the person has done enough to justify the relevant conditional premise, then there is no fallacy. Whether the person has done enough to justify that premise may, of course, be disputable, but that's where the dispute should focus.