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2.1: Logical Fallacies - Formal and Informal

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    Generally and crudely speaking, a logical fallacy is just a bad argument. Bad, that is, in the logical sense of being incorrect—not bad in sense of being ineffective or unpersuasive. Alas, many fallacies are quite effective in persuading people; that is why they’re so common. Often, they’re not used mistakenly, but intentionally—to fool people, to get them to believe things that maybe they shouldn’t. The goal of this chapter is to develop the ability to recognize these bad arguments for what they are so as not to be persuaded by them.

    There are formal and informal logical fallacies. The formal fallacies are simple: they’re just invalid deductive arguments. Consider the following:

    If the Democrats retake Congress, then taxes will go up.
    But the Democrats won’t retake Congress.
    Therefore, taxes won’t go up.

    This argument is invalid. It’s got an invalid form: If A then B; not A; therefore, not B. Any argument of this form is fallacious, an instance of “Denying the Antecedent.” (If/then propositions like the first premise are called “conditional” propositions. The A part is the so-called “antecedent” of the conditional. The second premise denies it. More use of this kind of vocabulary in Chapter 4.) We can leave it as an exercise for the reader to fill in propositions for A and B to get true premises and a false conclusion. Intuitively, it’s possible for that to happen: maybe a Republican Congress raises taxes for some reason (unlikely, but not unprecedented).

    Our concern in this chapter is not with formal fallacies—arguments that are bad because they have a bad form—but with informal fallacies. These arguments are bad, roughly, because of their content. More than that: their content, context, and/or mode of delivery.

    Consider Hitler. Here’s a guy who convinced a lot of people to believe things they had no business believing (because they were false). How did he do it? With lots of fallacious arguments. But it wasn't just the contents of the arguments (appeals to fear and patriotism, personal attacks on opponents, etc.) that made them fallacious; it was also the context in which he made them, and the (extremely effective) way he delivered them. Leni Riefenstahl’s famous 1935 documentary/propaganda film Triumph of the Will, which follows Hitler during a Nazi party rally in Nuremberg, illustrates this. It has lots of footage of Hitler giving speeches. We hear the jingoistic slogans and vitriolic attacks—but we also see important elements of his persuasive technique. First, the setting. We see Hitler marching through row upon row of neatly formed and impeccably outfitted German troops—thousands of them—approaching a massive raised dais, behind which are stories-high banners with the swastika on a red field. The setting, the context for Hitler’sspeeches, was literally awesome—designed to inspire awe. It makes his audience all the more receptive to his message, all the more persuadable. Moreover, Hitler’s speechifying technique was masterful. He is said to have practiced assiduously in front of a mirror, and it shows. His array of hand gestures, facial contortions, and vocal modulations were all expertly designed to have maximum impact on the audience.

    This consideration of Hitler highlights a couple of important things about the informal fallacies. First, they’re more than just bad arguments—they’re rhetorical tricks, extra-logical techniques used intentionally to try to convince people of things they maybe ought not to believe. Second, they work! Hitler convinced an entire nation to believe all sorts of crazy things. And advertisers and politicians continue to use these same techniques all the time. It’s incumbent upon a responsible citizen and consumer to be aware of this, and to do everything possible to avoid being bamboozled. That means learning about the fallacies. Hence, this chapter.

    There are lots of different informal logical fallacies, lots of different ways of defining and characterizing them, lots of different ways of organizing them into groups. Since Aristotle first did it in his Sophistical Refutations, authors of logic books have been defining and classifying the informal fallacies in various ways. These remarks are offered as a kind of disclaimer: the reader is warned that the particular presentation of the fallacies in this chapter will be unique and will disagree in various ways with other presentations, reflecting as it must the author’s own idiosyncratic interests, understanding, and estimation of what is important. This is as it should be and always is. The interested reader is encouraged to consult alternative sources for further edification.

    We will discuss 20 different informal fallacies, and we will group them into four families: (1) Fallacies of Distraction, (2) Fallacies of Weak Induction, (3) Fallacies of Illicit Presumption, and (4) Fallacies of Linguistic Emphasis. We take these up in turn.

    This page titled 2.1: Logical Fallacies - Formal and Informal is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Matthew Knachel via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.