If you think Christians, creationists and proponents of intelligent design are the only ones guilty of arguing from ignorance – think again. True, some say; “I do not see how such and such could happen by unguided material-processes, therefore God did it.” But others will say; “Even though I do not see how such and such could happen by unguided material-processes, science will eventually fill the gap and show that it does.” Neither of these of course are good arguments and it is probably safe to say; arguing from ignorance cuts across worldview boundaries. There is a particular form of this kind of fallacious reasoning I want to touch on in this post. Last week I was reading about Alvin Plantinga’s new book[i] where an atheist stated without qualification: “an absence of evidence is evidence of absence, and is more than good-enough reason for not believing something [sic].” But clearly such an unqualified statement is not true. It is in fact an argument from ignorance.

Absence of evidence simpliciter is not evidence of absence. W.L. Craig likes to use the following example; “If I say there is an elephant in the room, then you would expect to see a massive living creature shaped like elephant before you. If you do not see evidence of this sort, then you rightly infer there is no elephant present. But if I say there is a flea in the room; just because you do not see a small insect or have any other confirming evidence, you cannot rightly infer a flea does not exist in the room.” Of course you do not know there is one either! In Craig’s debate with Peter Slezak he put it another way: “The lack of knowledge for some entity X counts as positive evidence against X’s existence only in the case that if X did exist, then we should expect to see more evidence of X’s existence than what we do see.” The atheist Carl Sagan seemed to understand this as well. In the Demon-Haunted World he wrote, “This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence." [Emphasis added] In fact: One cannot infer the nonexistence of P merely from an absence of evidence for the existence of P unless one can rationally show there is evidence Q we should expect to see if P exists and yet Q is found to be absent.

To clarify, consider the following hypothetical: If we have no evidence for [the existence of prokaryotic microorganisms on Mars] P, can one rationally infer merely from the absence of evidence for P that P is false? Of course not! Nor can we infer P is true. If NASA develops an evidential test for P; say a series of probes which land on Mars to take soil samples looking for P (where a positive result is evidence Q) then we can see if P is true. If it is, we should expect to see Q. If we run the tests and do not find Q, then that is a defeater for P. But if NASA does not send the probes and run the tests for Q, we are no further along in knowing whether P is true or false. Our knowledge of P has neither gained nor lost warrant.

Now for some, the confusion lies with the word ‘absence.’ There is the obvious absence of evidence Q before investigation, which tells us nothing. Then there is the absence of Q after investigation, which acts as a defeater. When someone says “An absence of evidence for God is evidence of his nonexistence,” they might be implying a valid test for evidence Q has been tried and failed. If this is the case, have your interlocutor explicitly define Q and how a test for Q has failed to reveal what we would expect if God exists. This should bring to light the assumptions one who takes this kind of tacit approach might have. Otherwise, without any Q, we are dealing with an interpretation of the word ‘absence’ which puts it squarely in the category of fallacy.

After properly clarifying the need for relevant evidence, you might hear the common refrain: “I don’t need to give any argument or evidence for the nonexistence of God any more than I do for unicorns or flying spaghetti monsters.” But this should not get anyone off the hook and I have dealt with this tact here: http://www.apologetics.net/post/2010/03/15/The-Flying-Spaghetti-Monster.aspx. This opt-out excuse has nothing to do with the topic at hand. We have good evidence contrary to [the existence of unicorns and flying spaghetti monsters] P. They are well documented and discussed fictional entities. We therefore have a defeater for P and in the absence of any other evidence, good reason to reject P.

So in summary, absence of evidence simpliciter is not evidence of absence. If someone makes this statement in an unqualified way, politely ask them to define what sort of evidence they were expecting to see but didn’t. Otherwise, when it comes to knowledge:  Gaps, ignorance and unqualified absence of evidence do nothing to move the ball on the field of warrant.