Intro to Logic: Ad hominem and Appeal to the people

by Luke Muehlhauser on August 29, 2009 in Intro to Logic

Welcome to my course Intro to Logic (index). Here, we learn the basic skills of good thinking and their benefits in real life.

Last time, we discussed the difference between formal and informal fallacies. Today, we’ll discuss two of the formal fallacies.

All formal fallacies are a type of non sequitur (Latin for “it does not follow”), in which the conclusion is logically unrelated to the premises. For example: “I am tall, therefore all cars are green” or “Current science can’t explain consciousness, therefore God exists.”

Ad hominem

One of the most well-known fallacies is the ad hominem or “against the man” fallacy, which looks like this:

Hitler believed whites were superior to non-whites. But Hitler was a genocidal maniac. So, whites are not superior to non-whites.

It might be true that whites are not superior to non-whites. But this is a poor argument for that conclusion. In fact, it is a change of subject. The traits of the person supporting a proposition have nothing to do with whether or not the proposition is true. Consider:

Hitler believed the earth was round. But Hitler was a genocidal maniac. So, the earth is not round.

This is an example of ad hominem abusive. It takes the general form:

  1. Person A claims X.
  2. There is something objectionable about Person A.
  3. Therefore, X is false.

Politicians often use ad hominem arguments to discredit their opponent’s policies not by attacking the merits of the policy but by attacking the merits of their opponent.

Another type is ad hominem circumstantial, which points out that someone may be biased because of their circumstances:

Tobacco company representatives should not be believed when they say smoking doesn’t seriously affect your health, because they’re just defending their own multi-million-dollar financial interests.

Of course this may be true, but it is not a good argument that smoking seriously affects your health. This is another change of subject – from the effects of smoking to the circumstances of one of the debators.

Ad hominem tu quoque (“You too!”) points out that an arguer is inconsistent, for example:

You argue that stealing is wrong, but you were arrested for stealing in February 2008!

This is another change of subject. The arguer may indeed be a hypocrite, but this fact says nothing about whether or not stealing is wrong.

Luckily, ad hominem arguments are easy to spot because they are an abrupt shift of subject matter, from the topic at hand to a particular person.

Appeal to the people

An argumentum ad populum (“appeal to the people”) argues that “If many people believe something, it must be true.” But the popularity of a view is logically unrelated to its truth, and many extremely popular views of the past have been utterly false.

Examples, courtesy of Wikipedia:

Since 88% of the people polled believed in UFOs, they must exist.

It’s silly for you to claim that Hitler would not have attacked the United States if they hadn’t entered World War II. Everyone knows that he planned to conquer the world.

Brand X vacuum cleaners are the leading brand in America. You should buy Brand X vacuum cleaners.

(But there are some exceptions when the appeal to majority is not fallacious. For example, matters of social convention and democratic systems are defined by what is popular.)

I will not bore you by spending 20 weeks in a row discussing each of the common logical fallacies. Instead, we’re going to move on to other topics, and only occasionally explore other logical fallacies.

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{ 14 comments… read them below or add one }

IntelligentDasein August 29, 2009 at 6:55 pm

Keep up the good work. Logic is incredibly useful but very boring. I felt like tearing my ear drums out  everyday in Symbolic Logic.

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lukeprog August 29, 2009 at 8:22 pm

I’ll bet! When I get to symbolic logic and probabilistic logic (Cox, Jaynes), things are going to get tough. But I want to make sure there is a post at least briefly explaining every logic principle I am likely to reference in other posts on this blog. Even though these posts are unpopular, they will eventually add up to something quite useful, I think.

Keep up the tough work in Symbolic Logic!

I’ve always loved your nickname, btw. Not just clever but also a worthy goal.

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IntelligentDasein August 30, 2009 at 12:28 am

Thanks for noticing the Heidegger reference. I became a really big fan of his a few years ago. Continental is my strongest area in phil (I have a double major in history and philosophy, but I am not pursuing a Ph.D in phil until after I get my Ph.D in history *my specific history area is religious studies*).
If you want a good intro to logic book to suggest to your readers, I would recommend: Essentials of Logic Although it is a bit pricey, it is clear and understandable, concise, has good practice problems, great organization,  easy to read formulas and is more or less an entire logic class in a nutshell.

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Lorkas August 30, 2009 at 5:58 am

Here comes the proofreader:

Politicians often use ad hominem arguments to discredit their opponent’s policies not by attacking the merits of the policy but [by] attacking the merits of their opponent.

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Reginald Selkirk August 30, 2009 at 6:06 am

lukeprog: Even though these posts are unpopular,

They don’t get a lot of comments, because they are expository and uncontroversial. But I appreciate them for what they are.
 

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Reginald Selkirk August 30, 2009 at 6:08 am

There are times when ad hominem arguments might be appropriate. For example, if the opposition is using an argument from authority, it would be entirely appropriate to mention that their “authority” is a phoney who got his degree from a diploma mill.
 

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IntelligentDasein August 30, 2009 at 6:35 am

Reginald Selkirk: There are times when ad hominem arguments might be appropriate. For example, if the opposition is using an argument from authority, it would be entirely appropriate to mention that their “authority” is a phoney who got his degree from a diploma mill.

If it is relevant criticism, it is not an ad hominem. For example: It would not be an ad hominem attacking Kent Hovind’s biology creditentials when he makes a statement about evolution. It would be an ad hominem if you said he could not be taken seriously in science because he went to jail for tax evasion.

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Reginald Selkirk August 30, 2009 at 7:09 am

IntelligentDasein: If it is relevant criticism, it is not an ad hominem.

OK, so it’s built into the definition?
 

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lukeprog August 30, 2009 at 7:22 am

Lorkas: Here comes the proofreader

Thanks!

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lukeprog August 30, 2009 at 7:23 am

IntelligentDasein,

It might be appropriate, but it still doesn’t relate to the argument he might be making. Unless he’s making the argument that he did not go to jail for tax evasion.

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Reginald Selkirk August 30, 2009 at 4:40 pm

Another mistake i have run into is someone assuming that any insult is an ad hominem. It’s not.
“You’re ugly” is merely an insult.
“You’re ugly, therefore I am going to disregard your argument about moral truths” is an ad hominem fallacy. (Or so I believe. Correct me if I’m wrong.)
 

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lukeprog August 30, 2009 at 5:01 pm

Reginald,

Yeah, I’ve seen that, too! Good point.

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Kevin Craig September 25, 2009 at 11:16 pm

¿Shouldn’t this post be listed here:
http://commonsenseatheism.com/?p=1053

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lukeprog September 26, 2009 at 6:25 am

Ah yes, I was a bit behind on that. Thanks.

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