A lecture on fallacies

1. Mistakes in Reasoning
(Fallacies)

2. Inductive Arguments
• Our method for inductive arguments: memorize a few forms and
force inductive arguments into those forms, because we know how to
evaluate them.
• Another popular method is to memorize a long list of supposed
fallacies—so that we can avoid giving them and spot them when
others use them.
• In addition to requiring much more memorization, that approach has
another major drawback: either the supposedly fallacious form has
reliable instances in addition to unreliable ones (e.g., slippery slope)
or else the supposed fallacy is defined in a way that makes it useless
to learn (e.g., weak analogy).

3. Why learn the fallacies then?
• So that you will not be bullied by fallacy finders. You’ll know what
they’re accusing you of and you’ll be able to respond competently.
• So that you will become acquainted with more useful forms of
inductive arguments.

4. The ones we’ve already discussed:
• Look them up in our book (page numbers provided) and see if you
can say what kind of inductive argument (that we’ve already learned)
is exemplified by each of the following fallacies.
(1) Hasty Generalization (p.150)
(2) Weak Analogy (p.152)
(3) Accident (p.181)

5. Fallacies vs. Mistakes
• Some of the so-called fallacies are just factual errors rather than mistakes in
reasoning—that is, they are factual mistakes about premises rather than
mistakes of form. For example, a false dilemma (or alternatives) is an argument
with a false disjunction for a premise (see p.180).
• At least those are legitimate criticisms. Some ‘fallacies’ are not even that. For
example, false cause is the mistake of giving an argument with a false
conclusion of the form ‘A causes B’ (see p.154) Remember that we cannot
criticize an argument by saying it has a false conclusion!
• Several are just (unhelpfully) defined to be mistakes of form. For example
beside the point (or ignoratio elenchi—see p. 218) is an argument whose
premises don’t logically support its conclusion. That is bad, but any bad
argument can be described that way. Naming bad arguments doesn’t help
identify them. (Other examples: hasty conclusion, straw man, red herring)

6. Causal Fallacies—an example
• Post hoc fallacy is the mistake of thinking that because one thing precedes
another that the first caused the second. It’s an argument of this form:
A precedes B
So, A causes B.
• There are some good arguments like that and there are some poor ones.
(1) Clapping precedes the lights going out in the classroom. So, Clapping causes the lights going out.
(2) Increased ice cream sales precedes an increase in violent crime. So, increased ice cream sales
causes increased violent crime.
• You’d only have to observe the premise or argument (1) two or three times
before accepting its conclusion. However, decades of observing the
premise of argument (2) won’t convince you of its conclusion. What’s the
difference?

7. Post Hoc Arguments--Continued
• The key to discriminating good post hoc arguments from bad ones is not
memorizing the fallacy but knowing what criteria to use to tell the good ones
from the bad ones.
• The key for these arguments is (1) the existence of a plausible mechanism from A
to B and (2) the absence of a common cause of both A and B. (This really is just
our method for inductive arguments: for a certain form determine the criteria for
reliability.)
• The difference between premises and conclusions in post hoc arguments is
similar to the familiar distinction between correlation and causation. For
example, consider that size of police force correlates with amount of crime, and
see what causal conclusion follows.

8. Appeal to Ignorance
• This argument form takes a failure of evidence to be evidence of failure.
It has this form:
I have no good evidence for X.
So X is not true.
• There are good examples and bad ones. See if you can say which of the
following are reliable and which aren’t.
(1) I have no evidence that bigfoot exists, so it doesn’t.
(2) There is no evidence that a panther is in the room with me. So there is no
panther in the room with me.
(3) I have no evidence that you are innocent of setting the Packard plant on fire.
So you are not innocent of it (you did it).
(4) I have no evidence that the number of people in MI now is an odd number.
So there is an even number of people in MI now.

9. Appeals to Ignorance--Continued
• The key to telling the good from bad (in most cases) comes down to
whether the following criterion is met: I would have good evidence
for X if it were true.
• There are some examples that might be trouble for the criterion. Do
the following meet the criterion? Are they reliable or not?
(5) Juror at beginning of a trial: I have no evidence that the defendant is guilty,
so he isn’t.
(6) I have no evidence that there are invisible and otherwise imperceptible
trolls in the room. So, there aren’t any.

10. Ad Hominem
• An ad hominem argument is an attack against a person. For example, it is
the kind of argument made by someone who dismisses the Supreme
Court’s decision in the Hobby Lobby case by saying that the majority was
composed only of Catholic men. It ignores the arguments given and
instead focuses on characteristics of the people who gave them.
• That example is a kind of circumstantial ad hominem—an argument that
discredits a view or argument by focusing on the characteristics (or
circumstances) of the person who presents it.
• Circumstantial ad hominem aren’t defined to be poor criticisms, but they
probably seem obviously inappropriate to you. Now, consider the
interested party criticism of an appeal to authority that we considered
earlier (those arguments are less reliable when the authority is an
interested party); how is that criticism not just a circumstantial ad
hominem? Can we reconcile that interested-party criticism with the fact
that circumstantial ad hominems are poor criticisms?

11. Question Begging
• An argument begs the question, or is circular, when it assumes (as a premise) the very thing it tries to
prove (the conclusion). [Notice that we are using the term in a different way than reporters (&etc.)
who use ‘begs the question’ to mean ‘makes me wonder’.] Even thought the book distinguishes
question begging from circularity we will think of them as two ways of describing the same thing.
• Consider an example:
Joe: That salesperson said ‘I’ll be honest with you, this investment will make you filthy rich’ so I think we
should risk it.
Jan: He was lying just to get a commission.
Joe: No I trust him. He said he was being honest.
• Joe’s (extended) argument looks like this:
(1) The salesperson said he was being honest with us.
(2) So, he was being honest with us.
(3) He said the investment would make us rich.
(4) So, it will.
• The argument from (1) to (2) relies on an unstated premise, namely that the salesperson was being
honest with us (because his saying something isn’t evidence that it’s true unless he is being honest).
However, that’s just the conclusion of Joe’s (sub) argument; the unstated premise is the same as (2)!
So the argument

12. Question Begging--Continued
• Consider the simplest possible circular argument:
(1) You live on planet earth.
(2) So, you live on planet earth.
• Remember that there are just two ways to criticize arguments: bad form and false
premises. This argument is valid and has all true premises. So there is no good criticism
of the argument! Have I misled you about the possible criticisms of an argument or is
the simple argument above really flawless?
• I think the argument is, from a reasoning standpoint, flawless. You haven’t misled
anyone or deceived them if they come to accept your conclusion on the basis of this
circular argument. That (convincing someone) isn’t likely to happen, so we might say
that the argument is useless.
• Think about how confused someone would have to be to not believe the conclusion until
presented with the argument above. So, maybe giving the argument suggests you think
the recipient is confused. For that reason we might say that giving the argument is
insulting. Compare to the argument on the previous slide; when someone tries to
convince you of her honesty by claiming to be honest, it’s insulting right? Obviously,
you’d have to be really confused to come to believe someone is being honest on the
basis of her say-so.
• Uselessness and insult are not flaws of reasoning, but they are to be avoided anyhow.