Troythulu’s Top 20 Logical Fallacies #14

This, the 14th in my ongoing series of posts on errors in thought and reasoning, deals with what is referred to as the Reductio ad Absurdum, or ‘reduction to the absurd’ argument. This form of argumentation can be valid, in the case of showing an argument to be false by following it to its logical conclusion and showing this conclusion to be ridiculous, or as a form of Straw Man argument when fallacious: this is the False Reductio ad Absurdum. This post shall deal first with the latter.

This last is committed by misrepresenting an argument and forcing a conclusion that is absurd but which does not use the actual train of reasoning of the original argument, and is therefore not valid, such as the claim that if one is skeptical about the existence of alien visitors, one must also be skeptical of the Great Wall of China, since one has not seen it with one’s own eyes either, but this is fallacious as the requirements for evidence of the two are not the same. This argument ignores forms of evidence other than eyewitness testimony and also logical inference .

One of my favorite variations on this is the claim that because I am skeptical of psychic abilities I must therefore also be skeptical of dark matter and dark energy, and must therefore believe that, and I quote, “90% of the Universe must not exist because I haven’t seen those either.”

This argument ignores the fact that though there is some question as to what dark matter and dark energy are, they have been reliably observed through their effects on the visible portions of the universe around them, and the preponderance of evidence shows that they are real to a high degree of probability, pending further and better evidence. Clearly, unobservable Psi phenomena and directly invisible but otherwise observable astrophysical phenomena are not analogous.

There are valid uses of this line of argument as well, when the actual train of logic is retained without misrepresentation. Some of the claims of pseudohistory, for example, such as that the ancient Egyptians could not have built the pyramids without alien help, or that in order to know what a human skeleton looks like, they must have been given X-ray technology from ancient astronauts(arrgh! the stupid, it burns us! nasty tricksy hobbitses!), can aside from obvious also be demonstrated as absurd by applying the same train of logic to what the proponents of these ideas, mostly whites of European descent, dare not apply it to: medieval white Europeans, as follows–

[1] How could the medievals possibly have had the intelligence and imagination to invent such marvels of technology as the crossbow, stained glass windows, steel armor and weapons, and those massive cathedrals! They must have gotten their technology from aliens, because they were obviously too inept to come up with them on their own! And what’s up with all those religious apparitions during the Middle Ages (Gasp!)? Misinterpretations of alien astronauts, of course! They must also have gotten medical imaging technology from the aliens too, because the art of the Middle Ages has lots of skeletons in it, and how could those stupid, credulous medievals have possibly known what skeletons looked like without it?

Without belaboring the point, I think you get the idea.

This is generally a valid line of argument in formal logic and often used in mathematical proofs. When it is used in a specious manner, however, this is generally because of distortions in the argument’s logical structure, or the use of false analogies or hidden premises in the misrepresented argument.