Fallacious Reasoning

Fallacious Reasoning

Author: Matt Fanning

This packet is meant to provide a brief overview of critical thought and then cover in more depth four common methods of fallacious reasoning.

  1. Critical Thinking and Fallacious Reasoning
  2. Begging the Question
  3. Suppressed Evidence
  4. Slippery Slope
  5. Appeal to Ignorance
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Critical Thinking and Fallacious Reasoning

Critical thinking and rational thought consist of evaluating information and situations, filtering choices and then finding a conclusion that makes sense given the information. Patience is a requirement as the thinker will face uncertainties and answers will not always come quickly.

Begging the Question

"Begging the question" is a form of logical fallacy in which a statement or
claim is assumed to be true without evidence other than the statement or claim
itself. When one begs the question, the initial assumption of a statement is
treated as already proven without any logic to show why the statement is true
in the first place (Ordoveza, 2011).

(North 2006)

- Sandra


Begging the Question

(KPFOS, 2010)

Suppressed Evidence

Fallacy: Suppressed Evidence

Short Definition: Guilt by omission; a half-truth (Rubin, 2010)


Extended Definition: Failure to bring relevant evidence to bear on an argument; only give evidence to support while omitting negative evidence. (Rubin, 2010)


How this happens: The fallacy of suppressed evidence occurs when an arguer intentionally omits relevant data. This is a difficult fallacy to detect because we often have no way of knowing that we haven't been told the whole truth. Doing this makes the argument look stronger than it is (Rubin, 2010).


Where this happens:

  • This often happens in debates, especially in the topics of abortion, capital punishment, legalization of marijuana, legalization of prostitution, and the violence on TV.

Many advertisements do this fallacy by:

  • Blurting out at a thousand words a minute in a television advertisement (Carroll, 2009)
  • Approval of new drugs for public consumption, suppression of evidence is rampant in the known side effects (Carroll, 2009)
  • Micro print on drug labels (Ads inform us of a product's dangers only if required to do so by law)
  • Ads never state that a competitor's product is equally good.
  • A cogent (good) argument is one that presents all relevant evidence.

 - Nate

Suppressed Evidence

This video leaves out the evidence about how marijuana can affect a person’s development, lungs, judgment and immune system.

Slippery Slope

Assuming that one event will inevitably follow another without having sufficient evidence that it will.  The conclusion does not follow logically from the initial premise, so slippery slope is related to the irrelevant reason (non sequitor) fallacy (Cavender & Kahane, 2010).

The fallacy has the following basic form.

1. If A then B

2. If B then C

3. If C then D

4. Therefore A then D

After then initial premise no evidence is given as to why the the other premises and the conclusion follow.  In most uses D is a negative, or harmful, result.  Since we want to avoid D we should also avoid A, because A will uncontrollably lead to D.  With the the negative conclusion slippery slope also has the form of appeal to fear.

This can be a valid argument, but in the case of the slippery slope fallacy no good evidence is given for A leading to B and so on. Since the steps are not justified this is fallacious reasoning.


Slippery Slope

(Carolla, 2005)

Slippery Slope

It is important to remember that is possible for a slope to actually be slippery. There may be a precedent or logical deductions that lead down the slope.

(Tomorrow, 2011)

Appeal to Ignorance

Appeal to ignorance appealing to ignorance as evidence for something. (e.g., We
have no evidence that God doesn't exist, therefore, he must exist. Or: Because
we have no knowledge of alien visitors, that means they do not exist).
Ignorance about something says nothing about its existence or non-existence (Walker, 1997).

A fallacy based on the assumption that a statement must be true if it cannot be
proved false.

If I were adopted, then I would know about it by now.
I don't know that I'm adopted.
Therefore, I wasn't adopted.


 - Ryan

Appeal to Ignorance

(Appeal to Ignorance, 2006)


Glasbergen, R. (Artist). (2007). Penguin logic. [Web Graphic]. Retrieved from http://penguingeek.files.wordpress.com/2007/08/humor-penguin-logic.jpg

Ordoveza, P. (2011). Beg the question. Retrieved from http://begthequestion.info/

KPFOS. (2010). Retrieved from http://www.youtube.com/watch?v=dQZ3vSgwzy4&feature=related

North, R. (Artist). (2006). Trex beggin. [Web Graphic]. Retrieved from http://languagelog.ldc.upenn.edu  /myl/TRexBegging.png

Carroll, R. T. (2009). Critical Thinking mini-lesson 7 [Fallacy of Suppressed 
Evidence]. Retrieved from Skeptic's Dictionary website: http://www.skepdic.com/refuge/ctlessons/lesson7.htm

Rubin, S. (2010, July 7). Suppressed Evidence [Santa Rosa Junior College]. Retrieved from http://online.santarosa.edu/presentation/page/?28608

Cavender, N. M., & Kahane, H. (2010). Logic and contemporary rhetoric. (11th ed., p. 79). Belmont, CA: Wadsworth, Cengage Learning.

Carolla, A. (2005). Slippery slope guy [Web]. Retrieved from http://www.youtube.com/watch?v=Yl8FhFP05A0

Tomorrow, T. (Artist). (2011). Slippery slope. [Web Graphic]. Retrieved from http://politicalirony.com/wp-conten/uploads/2011/04/incline.png

Walker, J. (1997, July 27). List of common fallacies. Retrieved from http://nobeliefs.com/fallacies.htm

Appeal to ignorance [Web]. (2006). Retrieved from http://www.youtube.com/watch?v=sPfEBQ8v1uE