After a Twitter exchange with someone who was particularly challenged with regard to logic, I promised to post some basic lessons in logic (I’ve been teaching the subject for many years).

I began with a basic discussion of the nature of argumentation, which is one of the primary ways in which reasoning operates: it makes connections between statements. The second post concerned the evaluation of arguments, and how to tell if an argument works. In this piece I discuss fallacies, which are common mistakes in reasoning.

(And apologies for getting into teacher mode here…)

Bertrand Russell, a great logician
Bertrand Russell, a great logician


1) Ad Hominem (Latin for “against the person”).

Attacking the person making an argument, rather than attacking the argument itself.

Example: Congressman Stevens is a godless Marxist, so we can disregard his policy on the minimum wage.

The negative characteristics of the speaker have no bearing on the truth of what he or she is saying.

2) Appeal to Emotion.

Trying to persuade someone, not through arguments and good reasoning, but by trying to sway the person’s emotions.

Example: My client is the sole support of his aged parents. If he is sent to prison it will break their hearts, and they will be left homeless and penniless. You surely cannot find it in your hearts to reach any other verdict than “not guilty.”

The effect of the defendant’s prison sentence on his parents, heartbreaking as it might be, has no bearing on whether or not he actually committed the crime.

3) Appeal to Ignorance.

Claiming something to be true because it hasn’t been proven false.

Example: The 9/11 attacks were part of a CIA conspiracy. I know that because no one’s been able to prove that they weren’t.

It’s a staple of logic that you can’t prove a negative (how could you prove that unicorns or leprechauns don’t exist?); further, and more importantly, you could use this argument to prove absolutely anything: You can’t prove that X didn’t happen, so it must have. Plug in what you want for X. This means it’s vacuous or empty (doesn’t establish anything).

4) Appeal to Majority.

Claiming that something is true because a large number of people believe it to be true.

Example: Everyone I talk to thinks Congressman Smith is a crook, so I guess he must be one.

Just because a lot of people believe something of course has no bearing on whether it’s true.

5) Begging the Question (Circular Reasoning).

In an argument, the premises are supposed to support or establish the conclusion. In begging the question, the premises somehow rely upon the truth of the conclusion, so that the whole thing runs in a circle (premises support the conclusion, conclusion supports the premises).

In some instances of begging the question, the conclusion is just a restatement of one of the premises:

Example: Abortion is murder; therefore it’s wrong.

[Murder is by definition wrong, so this is circular.]

In other cases, one of the premises wouldn’t be true or acceptable unless the conclusion were true:

Example: That new student says that I am her favorite teacher, and she must be telling the truth, because she wouldn’t lie to her favorite teacher.

6) Diversion (Red Herring).

Changing the issue in the middle of an argument.

Example: John: Semiautomatic weapons should be banned because they have no other purpose than to kill civilians.
Jack: We already have laws against murder.

7) False Alternative (False Dilemma).

Eliminating relevant options; this is often in the form of an either/or.

Example: Either we cut social programs or we live with a huge deficit.

8) Non Sequitur.

When the premises don’t support, or are irrelevant to, the conclusion.

Example in Formal Logic: All New Yorkers are friendly people, so all friendly people are New Yorkers.

Example in Informal Logic: Professor Conard is the best teacher because he has brown hair.

Note that the second example could also be called a diversion. Many instances of diversion are non sequiturs, and vice versa.

9) Subjectivism.

Taking something to be true because you want it to be true; or, alternately, rejecting a claim or some evidence because you don’t want it to be true.

Example: You can argue all you want that democracy gives us only the illusion of control over the government, but I don’t buy it. I was brought up to believe in the democratic system.



Identify the fallacies committed in the following:

1. You may laugh about ESP, but you can’t prove it doesn’t work.

2. Barney is advocating for stronger gun control laws, but everybody knows he drinks too much. You shouldn’t listen to him.

3. Radio announcement: “Creatine is the number one workout supplement on the market. And that’s because of its popularity.”

4. Megan didn’t say that she loved the meal that I cooked for her. Therefore, she must have hated it.

5. How can you deny the existence of God?! Almost everybody around you is religious in some way or another.

6. Sarah is against the death penalty because she says it doesn’t deter crime and that it’s unjust. But what does she know? She’s never been the victim of a violent crime.

7. Bill Baxter deserves to be promoted to vice president. He has three small children, and just last week his wife was diagnosed with breast cancer.

8. Interviewer: Congressman, would you please explain why you voted for the bill that increases the taxes on middle- and lower-income citizens?
Congressman: Well, I believe in the truly American values of hard work, family, and children. And the foundation of these values is a sound economy, thriving businesses, national security, and honest leadership.

9. Paper is combustible, because it burns.

10. You can argue all you want that the two-party system in this country is corrupt, but I don’t buy it. I just think differently, that’s all.

Answers to follow in a subsequent post.

One thought on “Logic Part III: Informal Fallacies

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