Logic and Argumentation

This page explores the basics of logic and argumentation. Please also view the page on the different types of evidence used in arguments.

Argumentation is the staple of most, if not all, academic writing. Given the importance of argumentation, a writer should test and make certain that the point being argued is solid and well-founded rather than an unsupported statement, easily torn to shreds by a professor or colleague.

Ideally, a decent argument will be both valid and sound.

A valid argument is one where if all the premises are true, so that the conclusion necessarily follows. A non-valid argument is one where even if the premises are true, the conclusion does not necessarily follow. A sound argument is a valid argument with true premises, whereas an unsound argument has at least one false premise.

Example of a valid, sound argument:

Example of a valid, unsound argument:

Example of a non-valid argument:

The second argument is clearly not the case even though the line of argument makes logical sense. As for the third, the President of Wheaton College could have a great number of motivations for praying in chapel, including the one put forth in the non-valid argument, but this does not make him Catholic. Invalidity and unsoundness are often difficult to spot at first glance.

Logical Fallacies

Logical fallacies are logical mistakes made frequently and systematically enough to be formalized and given an official name. In order to make a sound, coherent argument, avoid them. Here is a (non-exhaustive) list of commonly occurring fallacies:

Begging the Question: the conclusion assumes or restates what is explicitly stated in the premise.

Appeal to Authority: citing the opinion of a person, usually a celebrity, who has no special expertise on the subject as a way of proving a point.

Appeal to the Populace: mentioning popular opinion as a way of convincing a reader instead of actually arguing.

Vagueness: using terms unclearly in a sentence or argument.

Equivocation: using a term in multiple ways in the same context.

Appeal to Ignorance: argues that, since a certain statement or point has not been shown to be false, it must be true—or vice versa.

Composition and Division: assuming that either the characteristics of one part define the whole or that a characteristic of the whole defines each individual part.

False Dilemma: reducing the options to two extremes.

Suppressed Evidence: making a conclusion without being aware of important background information.

Slippery Slope: claiming that one event will inevitably lead to a disastrous chain of consequences.

Conjunction: assuming that, since events A and B frequently occur together, they are the cause of one another.

Straw Man: intentional or unintentional misrepresentation of an opponent’s argument to make it an easier target to defeat.

Red Herring: diverting attention from the real point of an argument to something unrelated.

Reference: Weber, Ryan, and Allen Brizee. “Logical Fallacies.” Purdue University Online Writing Lab (OWL), 6 June 2018, owl.english.purdue.edu/owl/resource/659/03/.

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