There are two kinds of credulity here: credulity concerning (A) a metaphysical entity (such as God), for whose presence there is no empirical evidence but for which it is hard to define what counts as evidence of absence (especially since the entity itself is ill defined); and credulity concerning (B) physical entities (such as "weapons of mass production" in Iraq), for whose presence there is no empirical evidence other than fraudulently produced one, but for whose absence there is a great deal of evidence from credible sources, except that, given the size of the field of investigation (e.g., Iraq), it is hard to inductively rule out the possibility of their existence to zero.
In the case of (B), the non-zero but extremely low possibility fits into a commonly used (probabilistic) sense of the word improbable. In the case of (A), positive evidence for a categorically metaphysical entity, strictly speaking, cannot be provided, and claims for positive evidence for its manifestations in nature are either easily debunked hoaxes (like claims of miracles) or superfluous additions that explain nothing (like claims of Intelligent Design), even though it is not possible to empirically disprove the existence of such a metaphysical entity.
Appeal to ignorance thrives by seizing upon the "non-zero" part of "the non-zero but extremely low possibility" in (B) and the impossibility of empirically disproving a metaphysical entity in (A) and inflates them by undue fear or hope or both, making them seem more important than lack of positive evidence in either case -- at least in the minds of those who are credulous enough to buy such an appeal.
For all other people, there is no compelling reason to keeping on looking for either (A) or (B). That something is impossible to rule out due to the nature of the thing doesn't mean that there is a reason to believe that it will be found one day and is worth looking for it. -- Yoshie
* Critical Montages: <http://montages.blogspot.com/> * Monthly Review: <http://monthlyreview.org/> * Greens for Nader: <http://greensfornader.net/> * Bring Them Home Now! <http://www.bringthemhomenow.org/> * Calendars of Events in Columbus: <http://sif.org.ohio-state.edu/calendar.html>, <http://www.freepress.org/calendar.php>, & <http://www.cpanews.org/> * Student International Forum: <http://sif.org.ohio-state.edu/> * Committee for Justice in Palestine: <http://www.osudivest.org/> * Al-Awda-Ohio: <http://groups.yahoo.com/group/Al-Awda-Ohio> * Solidarity: <http://www.solidarity-us.org/>