What is a Logical Fallacy?

Illustration of three common logical fallacies and their definitions


A logical fallacy is an error in reasoning that renders an argument invalid. It is also called a fallacy, an informal logical fallacy, and an informal fallacy. All logical fallacies are nonsequiturs—arguments in which a conclusion doesn't follow logically from what preceded it. 

Clinical psychologist Rian McMullin expands this definition:

"Logical fallacies are unsubstantiated assertions that are often delivered with a conviction that makes them sound as though they are proven facts. ...Whatever their origins, fallacies can take on a special life of their own when they are popularized in the media and become part of a national credo"
(The New Handbook of Cognitive Therapy Techniques, 2000)

Examples and Observations

"A logical fallacy is a false statement that weakens an argument by distorting an issue, drawing false conclusions, misusing evidence, or misusing language."

(Dave Kemper et al., Fusion: Integrated Reading and Writing. Cengage, 2015)

Reasons to Avoid Logical Fallacies

"There are three good reasons to avoid logical fallacies in your writing. First, logical fallacies are wrong and, simply put, dishonest if you use them knowingly. Second, they take away from the strength of your argument. Finally, the use of logical fallacies can make your readers feel that you do not consider them to be very intelligent."

(William R. Smalzer, "Write to Be Read: Reading, Reflection, and Writing, 2nd ed." Cambridge University Press, 2005)

"Whether examining or writing arguments, make sure you detect logical fallacies that weaken arguments. Use evidence to support claims and validate information—this will make you appear credible and create trust in the minds of your audience."
(Karen A. Wink, "Rhetorical Strategies for Composition: Cracking an Academic Code." Rowman & Littlefield, 2016)

Informal Fallacies

"Although some arguments are so blatantly fallacious that at most they can be used to amuse us, many are more subtle and can be difficult to recognize. A conclusion often appears to follow logically and nontrivially from true premises, and only careful examination can reveal the fallaciousness of the argument.

"Such deceptively fallacious arguments, which can be recognized as such with little or no reliance on the methods of formal logic, are known as informal fallacies."

(R. Baum, "Logic." Harcourt, 1996)

Formal and Informal Fallacies

"There are two main categories of logical errors: formal fallacies and informal fallacies.

"The term 'formal' refers to the structure of an argument and the branch of logic that is most concerned with structure—deductive reasoning. All formal fallacies are errors in deductive reasoning that render an argument invalid. The term 'informal' refers to the non-structural aspects of arguments, usually emphasized in inductive reasoning. Most informal fallacies are errors of induction, but some of these fallacies can apply to deductive arguments as well.

(Magedah Shabo, "Rhetoric, Logic, and Argumentation: A Guide for Student Writers." Prestwick House, 2010)

Example of Logical Fallacies

"You oppose a senator's proposal to extend government-funded health care to poor minority children because that senator is a liberal Democrat. This is a common logical fallacy known as ad hominem, which is Latin for 'against the man.' Instead of dealing with the argument you preempt any discussion by basically saying, 'I cannot listen to anyone who does not share my social and political values.' You indeed may decide that you don't like the argument the senator is making, but it is your job to poke holes in the argument, not to engage in a personal attack."

(Derek Soles, "The Essentials of Academic Writing, 2nd ed." Wadsworth, 2010)  

"Suppose that each November, a witch doctor performs a voodoo dance designed to summon the gods of winter and that soon after the dance is performed, the weather, in fact, begins to turn cold. The witch doctor's dance is associated with the arrival of winter, meaning that the two events appear to have happened in conjunction with one another. But is this really evidence that the witch doctor's dance actually caused the arrival of winter? Most of us would answer no, even though the two events seem to happen in conjunction with one another.
"Those who argue that a causal relationship exists simply because of the presence of statistical association are committing a logical fallacy known as the post hoc propter ergo hoc fallacy. Sound economics warns against this potential source of error."
(James D. Gwartney et al., "Economics: Private and Public Choice," 15th ed. Cengage, 2013)
"The arguments in support of civic education are often seductive....
"Although we might emphasize different civic virtues, don't we all honor a love for our country [and] a respect for human rights and the rule of law.... Since no one is born with an innate understanding of these virtues, they must be learned, and schools are our most visible institutions for learning.
"But this argument suffers from a logical fallacy: Just because civic virtues must be learned, does not mean they can be easily taught—and still less that they can be taught in schools. Nearly every political scientist who studies how people acquire knowledge and ideas about good citizenship agrees that schools and, in particular, civics courses have no significant effect on civic attitudes and very little if any, effect on civic knowledge."
(J. B. Murphy, The New York Times, September 15, 2002)
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Nordquist, Richard. "What is a Logical Fallacy?" ThoughtCo, Jul. 31, 2021, thoughtco.com/what-is-logical-fallacy-1691259. Nordquist, Richard. (2021, July 31). What is a Logical Fallacy? Retrieved from https://www.thoughtco.com/what-is-logical-fallacy-1691259 Nordquist, Richard. "What is a Logical Fallacy?" ThoughtCo. https://www.thoughtco.com/what-is-logical-fallacy-1691259 (accessed June 2, 2023).