Welcome & welcome back to this series on logic! (Sorry I'm late; I usually post on Mondays, Wednesdays, & Fridays in the afternoon, but this week has been a little bit strange...)
The series so far:
Introduction:
Generalization Errors
Propositions & Arguments
Informal Fallacies:
Basic Ambiguity Fallacies
Personal Fallacies
Several Irrelevant Appeals
Generalization Errors
Question-Begging, Complex Question, Bifurcation
Argument From Ignorance, False Cause
...& as always, I encourage questions, comments, & suggestions.
Though the list can never be complete, the last of this summary of major informal fallacies lies beyond the Kos Antique Flourish....
Irrelevant Thesis (Ignoratio Elenchi).
A number of different forms exist for this fallacy, but generally someone commits this fallacy by arguing a point that's irrelevant to the subject at hand. A district attorney would commit this fallacy if he argued that simply because murder itself is terrible, then the particular defendant must be guilty of murder. (Ronald Reagan once made a similar argument as to why he refused to grant a pardon — based on the heinousness of an unrelated crime.) A particularly well-publicized example — one whereof many among us are reminded every December — was an 1897 editorial letter to a certain girl named Virginia. Now, as in Victorian times, how allegedly "dreary" the world would be without something (Santa Claus, in this example) has nothing to do with whether it really exists.
False Analogy.
Rhetorical analogies have great power to illustrate arguments. However, using analogies as-such in place of direct argument — that is, using argument by analogy — can produce an argument every bit as beside the point as a completely irrelevant tangent.
A political speaker may make a general statement that
Political change is as inevitable as the moon phases.
...& not thereby say anything too outrageous.
However, if he follows by arguing something along these lines:
Vote for Joe to replace Mayor Bob. After all, just like the phases of the moon, our city's progress must be allowed to advance without hindrance.
...he's making a rather weak argument for his case because it rests on an inappropriate analogy. The moon progressing through its phases, a natural sequence, requires no human input & a human being cannot hinder it, & thus it bears little resemblance to those changes in the life of a city which a mayor can alter.
Also, this argument seems to rest on equivocation in the meaning of the word "must", which can mean either "should" or "will" The speaker apparently leaves the precise meaning of this word obscure ("progress must be allowed"), no doubt to prop his weak analogy by a cheap verbal trick.
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Having now finished this broad overview of informal fallacies, I hope it proves helpful in naming what's wrong with many wrong arguments in the future!
As for the future of this series as a whole, many commenters have said that they're looking forward to delving into a general study of rhetoric, & perhaps advertizing appeals in particular. Since, even more so than with the past several diaries, this may require a bit of research that goes beyond what I myself have already done in detail — my own academic training was mostly in argument by citation rather than by rhetorical device! — I welcome suggestions as to subjects & approaches to take.