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From a psychological point of view, a fallacy is often defined as a mistake in reasoning or argumentation. Often fallacies are used for deceptive purposes and many of the informal fallacies described here are often used for the manipulation of opinion.

Many of these mistakes in reasoning occur so often they deserve special study.

This section investigates informal fallacies—those dependent upon language. An informal fallacy is one that arises from the content of an argument (the meaning what is declared), not a fallacy arising from the grammatical structure (the syntax of how the argument is expressed).

Our classification of fallacies, like that of I.M. Copi's, is arranged in terms of mistakes in reasoning arising from appeals to irrelevant factors as well as mistakes arising from unsupported assumptions.

Nevertheless, as Joseph says in his Introduction to Logic: “Truth may have its norm, but error is infinite in its aberrations, and they cannot be digested in any classification.”

Not all irrelevant appeals or unsupported assumptions are fallacies. Fallacies only occur in argumentative discourse. Thus, if no argument is present in a passage, no fallacy can be present.

Links to Lecture Notes
  • Nature of Fallacies
    Informal and formal fallacies and characterized in an introductory manner.


  • Ad Ignorantiam
    The argument from ignorance is characterized.

  • Ad Verecundiam
    The argument from irrelevant appeal to authority is characterized. 

  • Ad Hominem
    The argument by appealing to the character or circumstances of individual advancing a claim in order to determine its plausibility.
    Ad Hominem Example Exercises

  • Ad Populum
    The argument based upon what most or all people think or believe  is characterized.

  • Ad Misericordiam 
    The argument concerning the appeal to pity or a related emotion to gain the acceptance of a conclusion is evaluated.

  • Ad Baculum
    The argument based upon the appeal to force or threats in order to bring about the acceptance of a conclusion is often fallacious.

  • Ignoratio Elenchi
    The fallacy involving an irrelevant conclusion is discussed here as a "catch-all" category for fallacies not clearly  identifiable as any of the other fallacies of relevance.


  • Complex Question 
    The fallacy of complex question is discussed, and several typical examples are presented.

  • False Cause 
    The fallacy of false cause and its forms as non causa pro causa and post hoc ergo propter hoc is discussed with examples.

  • Petitio Prinicpii
    Petitio principii (viciously circular reasoning) is described and several examples are noted.
    Petitio Principii Example Exercises

  • Accident
    The fallacy of Accident is based upon the limited applicability of a “;glittering generality”;—from a generalization as a premiss, an atypical particular conclusion is claimed to follow. 

  • Converse Accident
    Converse Accident or hasty generalization is the fallacy of drawing a general conclusion based on one or several atypical instances.

  • Review Exercises
    Twenty-five self-test questions are presented with links to other exercises.

  • Fallacy Summary
    A summary of some common informal fallacies is outlined, and examples are provided.



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04.27.2017          © 1998-2017 Licensed under  GFDL

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