All the chewy goodness of fallacious reasoning in new fun-sized bites!
“Appeals to ignorance” or ad ignorantium arguments mistakenly draw a conclusion that a claim is true or false based on a lack of evidence for it. There are two forms this can take:
It has not been proved that P.
It has not been proved that not-P.
Consider these examples:
No one has ever proved that God exists.
Therefore, God does not exist.
No one has ever proved that God does not exist.
Therefore, God exists.
According to John Nolt et al., “These arguments suggest a false dichotomy: either our evidence for a claim is conclusive or the claim itself is false.” However, reality is not dependent upon our proofs. In other words, a claim can actually be true regardless of whether someone can prove it. In the absence of definitive proof, Nolt et al. suggest, “the rational approach is to weigh the available evidence, and, if the preponderance of the evidence favors one conclusion, to adopt that conclusion tentatively.”
It’s important to note, at this juncture, that 100% certainty is not necessary to accept a particular claim. As a matter of fact its acceptance is justifiable as long as our certainty is anywhere between 51-100%. Therefore, the suggestion to adopt a tentative conclusion based on the available evidence is good philosophy and should aid us as we trade in the marketplace of ideas.
The best way to point out someone’s fallacy is by using the Columbo tactic as laid out in the Funsized Tactics series. There are plenty of excellent resources available in book, e-book, or PDF format. Two good places to start, with regard to formal and informal fallacies, are A Preface to Philosophy by Mark B. Woodhouse and Schaum’s Outlines: Logic by John Nolt, Dennis Rohatyn, and Achille Varzi.
 John Nolt, Dennis Rohatyn, and Achille Varzi, Schaum’s Outlines: Logic, 2nd ed. (New York: The McGraw-Hill Companies, 2011), 203.