This chapter will be more difficult for students. By now you must understand the difference between questioning the truth of the premises and criticizing the reasoning even if the premises are true. Many of my students who have been unable to get this distinction straight and understand its implications begin to understand it better with coverage of the fallacies of Slippery Slope and Questionable Dilemma. It is fairly easy for them to understand that formally these are valid arguments, but that the premises are questionable.
Students who have studied very superficially often abuse the fallacies of Appeal to Ignorance and Suppressed Evidence. They think that any argument that sounds a little strange or one that might be offered by anyone ignorant of how the world works must be Appeal to Ignorance. They must be shown that this appeal is very specific in the premise (X has not been shown to be false, or X has not been shown to be true). Some students will think that any argument that does not address every possible tangential aspect related to the issue discussed must be Suppressed Evidence. No argument, of course, can address every possible aspect of an issue. You must understand that Suppressed Evidence is a very special case where the premise or premises are true and the argument apparently reasonable, but one very specific, known fact changes our perspective. The Crest toothpaste and Bush-Dukakis examples usually need to be gone over carefully.
The Begging the Question and Equivocation-Ambiguity fallacies are the most difficult to understand for many students. The God-divine inspiration example in the text works for most students. Students will confuse a circular argument with one that does not fully support its conclusion. For Equivocation-Ambiguity, it might help to think of some deliberate equivocations intended to be humorous. These are not difficult to find, because advertisers will often use them in attempting to be humorous to get our attention. For instance, in the summer of 1995 Jack-in-the-Box began using an advertisement for its "Really Big Chicken Sandwich." It involved a conversation between Jack and a rather slow-witted customer who wanted to see how big the chickens were that they used in the sandwiches. Jack explains, "No, it is not a really big chicken sandwich; it is a really big chicken sandwich." The customer professed to get it, but then asked how big the eggs are laid by these chickens. However, take a look at the Dunlop example again discussed on pages 6, 12-13, 53, and 108 as a reminder that there is a serious, exploitative side to this fallacy as well.
At this stage we can summarize the different categories
of fallacies. Please remember that knowing these categories is important
because they categorize exactly how fallacies are weak arguments.
If our ultimate goal is make more arguments stronger, then we need to know
exactly how some arguments are weak.
Fallacies of Relevance (All the fallacies in Chapter 4)
Here the focus is on the reasoning being weak. For every one of the fallacies in Chapter 4 we argue that even if the premises are true, the premises are not relevant to the conclusion, that the premises are not addressing the type of evidence relevant to accepting the conclusion.
Fallacies of Weak Induction (Some of the fallacies in Chapter 5)
Here the focus is also on the reasoning being weak, but with a very important difference, i.e., the premises ARE RELEVANT to the conclusion. To understand the difference between this type of fallacy and those in Chapter 4, you need to remember what we discussed about inductive reasoning in Chapter 3. There we distinguished between strong and weak inductive arguments. The point being that in the case of weak inductive arguments, even if the premises are true they give very weak inductive evidence for the conclusion. The implication here, however, is that the premises are at least "on track," they are presenting the kind of evidence that does support the conclusion. So, even in the case of a weak inductive argument, the premises are relevant; they are just NOT SUFFICIENT for a reliable or probable conclusion.
As an example, remember number #6, page 108. We classified this argument as a very weak induction, but the premises stating that John's and Jill's phones are busy are relevant to the possibility that they are talking to each other, which in turn is relevant to the possibility that they are having an affair. So this is some relevant evidence, but clearly very weak for the conclusion that they are having an affair.
Fallacies of Questionable Premise (Most of the arguments in Chapter 5)
The focus for these fallacies is not the reasoning; the reasoning is often valid. For these fallacies we argue that the premises are either false (as would be the case in Questionable Dilemma), questionable or unfair (as in the case of Slippery Slope), or presumptuous (as in the case of Questionable Analogy and Begging the Question). A presumption fallacy is one where we should not presume the premise is really providing any evidence at all. For instance, without direct evidence analogies are just introductions to arguments that help us understand a person's point of view.
Watch out for:
E.g.. Questionable Cause and Slippery Slope are very different fallacies, but both do involve causation. Questionable Cause is a fallacy of weak induction and always has a causal claim in the conclusion. Slippery Slope is a fallacy of questionable premise and has a causal claim in its key premise. Questionable Cause offers at least some evidence for its conclusion (a time sequence), but Slippery Slope offers no evidence for its controversial premise.
As a weak induction fallacy, some evidence is offered for the conclusion. But as noted above sometimes students just look at the name and think that any claim that they think is dumb must be an Appeal to Ignorance. For this fallacy, you must have a premise that either states, "Such and such has not been proven true," or "Such and such has not been proven false."
To argue for Suppressed Evidence, you must argue that the premise is true, not false as in the case of Straw Person. Also, you must be able to state that a specific fact is being suppressed, such as the following:
Conclusion: Buy Crest toothpaste.
Premise: Because it has fluoride.
Note that the premise is true and in general cites a good reason for accepting the conclusion. We know that a little fluoride is good to have in toothpaste, because it kills the bacteria that produce cavities.
But the specific fact that is suppressed is that all modern toothpastes have fluoride.
Conclusion: Don't vote for Bill Clinton.
Premises: Since Clinton has been president, heroin usage amongst teenagers has doubled.
This argument was used by Bob Dole during the 1996 presidential campaign. The specific fact that Dole suppressed was that the heroin usage amongst teenagers allegedly only went from .04% to .08% when Clinton was president! Many statisticians will argue that that change may be too statistically insignificant.
Bottom line. Be carefully claiming Suppressed Evidence. This approach to criticizing an argument is very different from arguing that a conclusion is hasty given the evidence (Fallacies of Weak Induction), or that the premises are not offering relevant evidence for the conclusion (Fallacies of Relevance).