Miscellaneous Informal Fallacies

Logic 101 – Lecture 4

Bob: Betty is a very trustworthy person, so I’m going to talk to her about my deepest darkest secrets.

Bruce: How do you know that?Bob: Well, Bill told me.Bruce: How do you know that Bill is a good

judge of character?Bob: Betty told me so.

Begging the Question

A.K.A. Circular logicAssuming a conclusion is true based on

premises that already assume that the conclusion is true.

This doesn’t work because if somebody doesn’t believe the conclusion, they also won’t believe the premises.The argument becomes basically useless

The people who already agree with the statement aren’t learning anything new

The people who disagree will find the whole argument invalid.

Begging the Question

Derek: All Scotsman love rugbyFinlay: But my uncle’s a Scotsman, and he hates

rugby!Derek: Well, no true Scotsman would hate rugby.

Jessica: Why do we celebrate Mother’s Day?Kelly: Because we need to recognize the

compassion of mothers everywhere!Jessica: My mom was drunk five days a week and

called me a whore for wearing shorts above my knees.

Kelly: She wasn’t a real mother, then.

No True Scotsman

Actively redirecting an understood definition mid-argument.

A Scotsman is primarily defined as a person who hails from ScotlandDerek changes his definition of “Scotsman” to

a person who fits his qualifications of the title rather than the agreed-upon definition

Not a fallacy if the arguer is in a position to define the statement in question, or the opposite statement is ill-informed.

No True Scotsman

Coach Dan: A split end is a receiver who lines up apart from the offensive line, but still on the line of scrimmage.

Kurt: But Charlie said he’s a split end, and he lines up behind the line of scrimmage!

Coach Dan: He’s not a true split end- that makes him a flanker.

No True Scotsman

Sen. Dramatic: Either we boost funding to fight the War on Drugs or our nation will crumble as our children become victims of drug addiction, tearing the fabric of our society and crippling our nation for generations.

Joey: Either you’re coming to watch my game, or you don’t really love me and I’m breaking up with you.

False Dichotomy

Attempting to establish a conclusion, P, by falsely assuming that the only two options are P and Q (and Q is clearly false or unacceptable).

The arguer ignores the other option(s), making this a fallacy

This is not a fallacy if there are, in fact, only two options.

False Dichotomy

Mother: Why didn’t you like the sandwich I made you?

Daughter: It was terrible! You put chocolate ice cream in lettuce and covered it with ketchup!

Mother: But you like all of those things!

Father: Son, why aren’t you eating that food coloring I poured you?

Son: Because food coloring doesn’t taste good.

Father: But all the candy you eat has food coloring in it!

Composition / Division

Misattributing properties of the parts to the whole (composition) or properties of the whole to the parts (division)

Case in point: I’m sure as heck not putting ketchup on my ice cream, even though I like both.

Not a fallacy if they actually share properties (duh)

Composition / Division

Dr. Hippie: The cancer rate of our city is significantly higher than the surrounding cities. As our city is the only one to contain a factory emitting dihydrogen monoxide into the atmosphere, we can conclude that this dangerous chemical is causing cancer.

Texas Sharpshooter

Derived from a joke about a Texan who shoots at a random point on the side of a barn, and then draws a bulls-eye around the bullet holes to make it look like he’s a great shot.

You’re not allowed to pull causal relationships out of data without proper procedureWhich is related to the clustering illusion - why

people see Jesus in their toast.Not a fallacy if the data in question was

derived with an experimental goal in mind

Texas Sharpshooter

From Wikipedia:Loki is a trickster god in Norse mythology, who, legend

has it, once made a bet with some dwarves. It was agreed that the prize, should Loki lose the wager, would be his head. Loki lost the bet, and in due time the dwarfs came to collect the head which had become rightfully theirs. Loki had no problem with giving up his head, but he insisted they had absolutely no right to take any part of his neck. Everyone concerned discussed the matter. Certain parts were obviously head, and certain parts were obviously neck, but neither side could agree exactly where the one ended and the other began. As a result, Loki keeps his head indefinitely, but loses his lips, as they were clearly part of the head which the dwarfs had complete rights to.

Loki’s Wager

The unreasonable insistence that, since a concept cannot be defined, it cannot be discussed.In effect, the opposite of a hasty generalization

(Lecture 3)This is generally accomplished by blending

different definitions of the concept in questionIf you really want to be hyper-semantic, you can

argue that almost everything has no definite boundaries, meaning we can’t define anything.

Loki’s Wager

Suspicious wife: If John was having an affair with his secretary, then he would have her go with him on his next business trip. He just called today to tell me that his secretary going with him, so he must be having an affair.

Trusting wife: If John is having an affair with his secretary, then he would have her go with him on his next business trip. He’s not having an affair with her, so she won’t be going with him on that trip.

Affirming the Consequent /Denying the Antecedent

Actually a formal fallacy, but the most commonly-committed one.

A conditional (if P, then Q) only has two rules that can operate on itIf it’s your birthday, then I’ll bake you a cake.

It’s your birthday. Therefore, I will bake you a cake.I did not bake you a cake. Therefore, it’s not your

birthday

While the other two operations sound reasonable, they are actually invalid.There are plenty of reasons why John’s secretary

would (or wouldn’t) come with him that have nothing to do with an affair

Affirming the Consequent /Denying the Antecedent

An argument is valid if and only if the following is trueIf the premises are true, the conclusion must

be trueSome things that may make an argument

invalidThings that are irrelevant to the argument

Where the argument came from, who supports the argument

Things that aren’t good reasoningAssuming causal relationships, falsely assuming

expertiseThings that just don’t go anywhere

Redefining key terms, making assumptions out of randomness

Informal Fallacies – Review

The most important part about being a good logician is knowing how to identify fallacies, not just knowing what they are.

Remember the exceptions to the rules just as well as the rules themselves.

Catch yourself making mistakes in reasoning. Keep yourself honest (and right).

Remember that nobody is immune to errors in reasoning.

And most importantly…

Informal Fallacies – Review

IF THE LOGIC ISN’T THERE, THEN THE

ARGUMENT IS INVALID

No matter how scary the idea is, no matter how much you like/respect/fear the person making the argument, no matter how much you’d really like things to work the other way. If it’s wrong, it’s wrong.

Informal Fallacies - Review