26. December 2011 22:03
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 that P. If NASA develops an evidential test for that 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 that P. But if NASA does not send the probes and run the tests for Q, we are no further along in knowing that P or that ~P. Our knowledge of that P has neither gained nor lost warrant.
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.
[i] Where the Conflict Really Lies: Science, Religion, and Naturalism by Alvin Plantinga (Dec 9, 2011)