Monday, July 5, 2010

What is a Fallacy in Logic?

Definition of a fallacy: A fallacy is a type of argument that may seem to be correct but which proves, upon examination not to be so. In other words, it is an error in logic - the place where the person makes a mistake in their thinking.

Fallacies can be divided essentially into to two broad groups: formal fallacies and informal fallacies. Formal fallacies are most conveniently discussed in connection with certain patterns of valid inference to which they bear a superficial resemblance. Informal fallacies are errors in reasoning into which we may fall either because of carelessness or because of inattention to our subject matter or through being misled through some ambiguity in language used to formulate our argument. We may divide informal fallacies into fallacies of relevance and fallacies of ambiguity.

By fallacies of relevance, we are talking about those fallacies common to all arguments. The committal of a fallacy of relevance (except for the fallacy of petitio principii, i.e., begging the question) involves a circumstance where the premises of an argument are logically irrelevant to, and therefore are incapable of establishing the truth of their conclusion.


Editor's note: There are a few good resources that one can pick up in new and used bookstores. Here are just a few.

  • Nathaniel Bluedorn and Hans Bluedorn, The Fallacy Detective: Thirty-six Lessons on How to Recognize Bad Reasononing. Muscatine, IA: Christian Logic and Trivium Pursuit, 2003.
  • Irving M. Copi and Carl Cohen, Introduction to Logic (13th Edition).
  • Norman Geisler and Ronald Brooks, Come Let us Reason: An Introduction in Logical Thinking. Grand Rapids: Baker Books, 1990.
  • William Weston, A Rulebook for Arguments. Indianapolis, IN: Hackett Publishing, 2009.


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