Taking the Bad with the Good
Most of us are guilty from time to time of producing or falling prey to weak arguments. It may be that we just don't think--everyone has their moments of weakness, after all--or that we fail to observe something important. On the other hand, it may be that we are taken in by an argument that appears compelling, but whose beauty is only skin deep. Whatever the reason, it's a fact that we can't avoid.
Framing the Fallacies
Fallacious reasoning, like all turkeys, can be carved up in different ways. In this section, I briefly describe several different ways of classifying the fallacies before introducing the framework that will structure the remaining discussion in this chapter.
• Psychological Fallacies: these have the arguer as their source.
• Logical Fallacies: these flaws are grounded in the structure of the argument.
• Material Fallacies: these are fallacies that concern the content, or subject matter, of an argument.
• Fallacies of Form
• Fallacies of form are argumentative flaws typically discovered during formal analysis. These are flaws in the inferential structure of an argument, i.e., the relationships among the constituent propositions that convey truth from the premises to the conclusion. Because of this, fallacies undermine the support offered by reasons for conclusions. Since these are flaws in the structure of arguments and argument structure is repeatable, examination of it requires abstraction from specific details of a given argument. At this abstract level, the focus is on the structure of constituent propositions and the relations between propositions, considered structurally. The fallacies I identify in this section are argumentative moves that generally vitiate support by breaking or weakening the connections between constituent propositions.
• Affirming the Consequent: As we observed in Chapter 6, modus ponens is a valid inference pattern. This pattern can be symbolized as follows: If A, then B; A; therefore, B. A superficially similar pattern results by switching the second occurrences of 'A' and 'B' around, yielding: If A, then B; B; therefore, A. This, however, is an invalid move known as affirming the consequent. The second assertion in this pattern affirms B, which is the consequent in the conditional contained in the first assertion. This fallacy is grounded in the conditional, and it qualifies as a fallacy because it does not respect the way in which the conditional channels truth. That is, true conditionals guarantee the truth of the consequent given the truth of the antecedent, but this guarantee does not go in the opposite direction. (Put another way: antecedents are sufficient conditions on consequents, but consequents are not sufficient conditions on antecedents.) Thus, given a true conditional and a true consequent, the antecedent might turn out to be false, a fact that establishes this pattern as invalid. Consider: "If I'm in my office, my lights are on." If this is true and you see my lights on, you cannot infer that I'm in there, since it could be the janitor who has flipped on the lights. (Modus ponens, by contrast, could be called affirming the antecedent.)
Example: If Herman is deathly ill, then he'll be at home. Hey, Herman is at home. Therefore, he must be deathly ill. (This said of a guy who likes to stay home.)
• Denying the Antecedent: We also saw in Chapter 6 that modus tollens is a valid inference pattern. This pattern can be symbolized as follows: If A, then B; not B; therefore, not A. A superficially similar pattern results by switching the second occurrences of 'B' and 'A' around, yielding: If A, then B; not A; therefore, not B. This, however, is an invalid move known as denying the antecedent. The second assertion in this pattern denies A, which is the antecedent in the conditional contained in the first assertion. As with affirming the consequent, this fallacy is grounded in the fact that the pattern fails to respect the logic of the conditional. True conditionals guarantee the falsity of the antecedent given the falsity of the consequent, because the consequent is a necessary condition on the antecedent. However, the antecedent is not a necessary condition on the consequent, and so this pattern does not go in the opposite direction. (Modus tollens, by contrast, could be called denying the consequent.)
Example: If I get a "B" in this class, then there is justice in the universe after all. What?!?! I got a "C"? There is no justice!
Explanation:
