A Philosopher's Blog

42 Fallacies for Free in Portuguese

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on October 28, 2014

Thanks to Laércio Lameira my 42 Fallacies is available in Portuguese as a free PDF.

42 Falacias

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Education & Negativity Bias

Posted in Universities & Colleges by Michael LaBossiere on March 3, 2014
StateLibQld 1 113036 Cartoon of students recei...

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In general, people suffer from a wide range of cognitive biases. One of these is known as negativity bias and it is manifested by the tendency people have to give more weight to the negative than to the positive. For example, people tend to weigh the wrongs done to them more heavily than the good done to them. As another example, people tend to be more swayed by negative political advertisements than by positives ones. This bias can also have an impact on education.

A colleague of mine asks his logic students each semester how many of them are planning on law school. In the past, he had many students. Now, the number is considerably less. Curious about this, he checked and found that logic had switched from being a requirement for pre-law to being a mere recommendation. My colleague noted that it seemed irrational for students who plan on taking the LSAT and becoming lawyers to avoid the logic class, given that the LSAT is largely a logic test and that law school requires skill in logic. He made the point that students often prefer to avoid the useful when it is not required and only grudgingly take what is required. We discussed a bit how this relates to the negativity bias: a student who did not take the logic class when it was required would be punished by being unable to graduate. Now that the class is optional, there is only the positive benefit of a likely improvement on the LSAT and better performance in law school. Since people weigh punishments more than rewards, this behavior makes sense—but is still irrational. Especially since many of the students who skip the logic class will end up spending money taking LSAT preparation classes that will endeavor to spackle over their lack of skills in logic.

I have seen a similar sort of thing in my own classes. At my university, university policy allows us to lower student grades on the basis of a lack of attendance. We are even permitted to fail a student for excessive absences. While attendance is mandatory in my classes, I do not have a special punishment for missing class. Not surprisingly, when the students figure this out around week three or four, attendance plummets and then stabilizes at a low level. Before I used BlackBoard for quizzes, exams and for turning in assignments and papers, attendance would spike back up for days on which something had to be done in class. Since students can do their work via BlackBoard, these spikes are gone. They are, however, replaced by post-exam spikes when students do badly on the exams because they have not been in class. Then attendance slumps again. Interestingly, students often claim that they think the class is interesting and useful. But, since there is no direct and immediate punishment for not attending (just a delayed “punishment” in terms of lower grades and a lack of learning), many students are not motivated to attend class.

Naturally, I do consider the possibility that I am a bad professor who is teaching a subject that students regard as useless or boring. However, my evaluations are consistently good, former students have returned to say good things about me and my classes, and so on. That said, perhaps I am merely deluding myself and being humored. That said, it is easy enough to draw an analogy to exercise: exercise does not provide immediate rewards and there is no immediate punishment for not staying fit—just a loss of benefits. Most people elect to under-exercise or avoid it altogether. This, and similar things, does show that people generally avoid that which is difficult now but yields lasting benefits latter.

I have, of course, considered going to the punishment model for my classes. However, I have resisted this for a variety of reasons. The first is that my personality is such that I am more inclined to want to offer benefits rather than punishments. This seems to be a clear mistake given the general psychology of people. The second is that I believe in free choice: like God, I think people should be free to make bad choices and not be coerced into doing what is right. It has to be a free choice. Naturally, choosing poorly brings its own punishment—albeit later on. The third is the hassle of dealing with attendance: the paper work, having to handle excuses, being lied to regularly and so on. The fourth is the fact that classes are generally better for the good students when the students who do not want to be in class elect to not attend. While I want everyone to learn, I would rather have the people who would prefer not to learn not be in class disrupting the learning of others—college is not the place where the educator should have to spend time dealing with behavioral issues in the classroom. The fifth is I prefer to reduce the amount of lying that students think they have to engage in.

In terms of why I have been considering using the punishment model, there are three reasons. One is that if students are compelled to attend, they might very well inadvertently learn something. The second is that this model is a lesson for what the workplace will be like for most of the students—so habituating them to this (or, rather, keeping the habituation they should have acquired in K-12) would be valuable. After all, they will probably need to endure awful jobs until they retire or die. The third is that perhaps many people lack the discipline to do what they should and they simply must be compelled by punishment—this is, of course, the model put forth by thinkers like Aristotle and Hobbes.

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30 More Fallacies in Print

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on March 15, 2013


Now available in print on Amazon and other book sellers.

30 Fallacies is a companion book for 42 Fallacies. 42 Fallacies is not, however, required to use this book. It provides concise descriptions and examples of thirty common informal fallacies.

Accent, Fallacy of
Accident, Fallacy of
Amphiboly, Fallacy of
Appeal to Envy
Appeal to Group Identity
Appeal to Guilt
Appeal to Silence
Appeal to Vanity/Elitism
Argumentum ad Hitlerum
Complex Question
Confusing Explanations and Excuses
Cum Hoc, Ergo Propter Hoc
Equivocation, Fallacy of
Fallacious Example
Fallacy Fallacy
Historian’s Fallacy
Illicit Conversion
Incomplete Evidence
Moving the Goal Posts
Oversimplified Cause
Overconfident Inference from Unknown Statistics
Pathetic Fallacy
Positive Ad Hominem
Proving X, Concluding Y
Psychologist’s fallacy
Reification, Fallacy of
Texas Sharpshooter Fallacy
Victim Fallacy
Weak Analogy

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Icelandic Logic

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on June 8, 2012

Valgarður Guðjónsson is presenting some of my fallacy material in Icelandic, thus helping to expand the empire of reason.

If you can read Icelandic (or not), you can check out his first blog on the subject: http://blog.eyjan.is/valgardur/2012/06/07/rokvillur/

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42 Fallacies in Ukrainian

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on August 16, 2011

If you happen to read Ukrainian, the text of my 42 fallacies have been translated and posted at http://webhostinggeeks.com/science/fallacies-nizkor-ua by Galina Miklosic .

Pop Up Fallacies

Posted in Reasoning/Logic by Michael LaBossiere on October 20, 2010
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Proving X, Concluding Y

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on September 22, 2010


This fallacy occurs when a conclusion is drawn from evidence that does not support that conclusion but another claim.  The form of this reasoning is as follows:

  1. Evidence for claim X is presented.
  2. Conclusion: Y

While all fallacies are such that the alleged evidence provided in the premise(s) fails to adequately support the conclusion, what distinguishes this fallacy is that the evidence presented actually does provide support for a claim. However, it does not support the conclusion that is actually presented.

This fallacy typically occurs when the evidence for X seems connected or relevant to Y in a logical way, but actually is not. It is this seeming relevance or connection that lures the victim into accepting the conclusion. As such, this differs from fallacies in which the victim is lured to the conclusion by an emotional appeal.

Obviously, this fallacy (like all fallacies) is a case of non-sequiter (“does not follow”) in which the conclusion does not logically follow from the premises. However, this specific sort of mistake is common and interesting enough to justify giving it its own name and entry.

Example #1

“I am troubled by the reports of binge drinking by college students. According to the statistics I have seen, about 19% of college students are binge drinkers and this leads to problems ranging from poor academic performance to unplanned pregnancies. Since people often drink in response to pressure, this shows that professors are putting their students under too much pressure and hence need to make their classes easier.”

Example #2

“Our product testing revealed that 60% of the people on Acme Diet Master reported that they felt less hungry when using the product.  This shows that 60% ate less when using our product. I think we have our next big product!”

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Illicit Inductive Conversion

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on September 20, 2010


This fallacy occurs when someone makes illicit use of the conversion rule from categorical logic in the context of inductive reasoning.  This fallacy has the following form:

  1. Premise 1: P% (or “some”, “few”, “most”, “many”, etc.) of Xs are Ys.
  2. Conclusion: Therefore P% (or “some”, etc.) of Ys are Xs.

In deductive logic, conversion is a rule that allows the subject and predicate claims of a categorical claim to be exchanged. In categorical logic there are four sentence types (All S areP , No S are P, Some S are P, and Some S are not P) and this rule applies correctly to two of them: No S are P and Some S are P. A conversion is legitimate when the converted claim logically follows from the original (and vice versa). Put another way, the rule is applied correctly when its application does not change the truth value of the claim.

For example, “No cats are hamsters” converts legitimately to “no hamsters are cats.” Interestingly, “some dogs are huskies” converts correctly to “some huskies are dogs”, at least in categorical logic. In categorical logic, “some” means “at least one.” Hence, “at least one dog is a husky” is converted to “at least one husky is a dog.” In this case, the inference from one to the other is legitimate because it is made in the context of categorical logic.

The illicit use of conversion is, not surprisingly, an error.  This error occurs in two ways. The first is when the rule is applied incorrectly in the context of categorical logic: if conversion is applied to an All S are P or a Some S are not P claim, then the rule has been applied improperly.  This can be easily shown by the following examples. While it is true that all dogs are mammals, the conversion of this claim (that all mammals are dogs) is not true.  As another example, the claim that some dogs are not huskies is true while its conversion (that some huskies are not dogs) is false.

The second occurs when the conversion rule is applied outside of the context of categorical logic.  To be specific, it occurs in contexts in which “some” and comparable terms such as “few”, “most”, “many” and so on are not taken to mean “at least one.” This is the sort of error that defines this specific fallacy. For example, to infer that most people who speak English are from Maine because most people from Maine speak English would be an obvious error.  This is because “most” in this context is not taken to mean “at least one.”

Not surprisingly, people generally do not make such obvious errors in regards to conversion. However, people do fall victim to conversions that seem plausible. For example, when people hear that a medical test for a heart condition is 80% accurate they might be tempted to infer that 80% of those who test positive have the condition. However, to convert “80% of those who have the condition will test positive” (that is what it means for a test to be 80% accurate) to “80% of those who test positive have the condition” is an illicit use of conversion.

Example #1

“Very few white men have been President of the United States. Therefore very few Presidents have been white men.”

Example #2

“A fairly small percentage of automobile accidents involve drivers over 70. Therefore a fairly small percentage of drivers over 70 are involved in automobile accidents.”


“Most conservatives are not media personalities on Fox News. Therefore, most of the media personalities on Fox News are not conservative.”

Example #4

“Most wealthy people are men so most men are wealthy.”

Example #5
“Most modern terrorists are Muslims, therefore most Muslims are terrorists.”

Example #6

“Most modern terrorists are religious people, therefore most religious people are terrorists.”

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Fallacy Video 3

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on September 14, 2010

Fallacy Video 2

Posted in Philosophy, Reasoning/Logic by Michael LaBossiere on September 13, 2010