Logic & Critical Thinking Series Part 4: Logical Fallacies

Form:

1. If P, then Q
2. Q
3. ? P ?

Example:

"If she's a doctor, she has a medical degree."
"She has a medical degree."
"Therefore, she's a doctor."

Why it fails: She could be a researcher, professor, or retired. Having a degree doesn't mean she's practicing medicine.

Form:

1. If P, then Q
2. Not P
3. ? Not Q ?

Example:

"If it's raining, the streets are wet."
"It's not raining."
"Therefore, the streets aren't wet."

Why it fails: The streets could be wet from sprinklers, a burst pipe, or car washing. Rain isn't the only cause.

Form:

1. All A are B
2. All C are B
3. ? All A are C ?

Example:

"All cats are mammals."
"All dogs are mammals."
"Therefore, all cats are dogs."

Why it fails: The middle term "mammals" isn't distributed (discussed in full). Cats and dogs share a category but aren't identical.

Abusive: Directly attacking the person's character.

"You can't trust John's argument about tax policy—he's a college dropout."

Circumstantial: Claiming someone's position is biased by their circumstances.

"Of course she supports higher teacher salaries—she's a teacher!"

Tu Quoque (You Too): Deflecting criticism by pointing to the critic's behavior.

"You criticize my smoking? You used to smoke yourself!"

Pattern: Misrepresent your opponent's argument, then attack the weaker version.

Example:

Person A: "We should have stricter food safety regulations."
Person B: "So you want the government to control everything we eat? Next they'll be telling us we can only eat vegetables!"

The Distortion: Person A argued for food safety, not total government control. Person B attacks an extreme position Person A never held.

Pattern: Introduce an unrelated topic to divert attention from the original issue.

Example:

Q: "Senator, what about your voting record on environmental issues?"
A: "I'm glad you asked about my record. You know, I've always supported our troops and believe strongly in national defense..."

The name comes from the practice of using smoked fish to train hunting dogs—the strong scent would lead them off the trail.

Substituting emotional manipulation for logical argument:

• Appeal to Fear (Ad Metum): "Vote for me or disaster will strike!"
• Appeal to Pity (Ad Misericordiam): "He shouldn't go to prison—think of his children!"
• Appeal to Flattery: "A sophisticated person like you surely agrees..."
• Appeal to Spite: "They look down on you—show them they're wrong!"

Emotions are legitimate parts of moral and practical reasoning, but they can't substitute for evidence and logic.

Pattern: Using a word with different meanings in the same argument.

Classic Example:

"The end of a thing is its perfection."
"Death is the end of life."
"Therefore, death is the perfection of life."

The Trick: "End" means "purpose/goal" in premise 1 but "termination" in premise 2. The argument equivocates on this word.

Modern Example:

"I have the right to watch whatever I want on TV."
"Therefore, I am right to watch whatever I want on TV."

"Right" (entitlement) ? "right" (correct/moral).

Pattern: Presenting only two options when more exist.

"You're either with us or against us."
"America: love it or leave it."
"Either we ban all cars or accept traffic deaths."

The Reality: Most issues have a spectrum of positions—neutrality, partial agreement, qualified support, etc. Black-and-white thinking oversimplifies complex issues.

Pattern: The conclusion is assumed (often disguised) in the premises.

Example:

"God exists because the Bible says so."
"The Bible is true because it's God's word."

Subtle Example:

"Free markets are best because they're unregulated, and unregulated markets are most efficient."

Note: The phrase "begs the question" is often misused to mean "raises the question." Technically, it refers specifically to this circular fallacy.

Pattern: Claiming that one step will inevitably lead to extreme consequences without justifying the causal chain.

Example:

"If we legalize marijuana, next people will want to legalize cocaine, then heroin, and soon all drugs will be legal and society will collapse."

When It's Not Fallacious:

• When there's evidence for the causal chain
• When institutional/psychological mechanisms make progression likely
• When historical examples support the pattern

The slippery slope is fallacious only when the chain of causation is assumed rather than demonstrated.

Fallacious when the authority is:

• Unqualified: "A celebrity endorses this vitamin—it must work!"
• Outside expertise: "A physicist says the economy will crash."
• Biased: "The tobacco company's scientist says smoking is safe."
• Speaking against consensus: A lone expert against thousands.

Legitimate Appeal: Citing experts within their field of expertise, especially when there's consensus. We can't verify everything ourselves—reasonable reliance on qualified authorities is rational.