As you write and review logical arguments, be aware of logical fallacies, or common errors in thinking that can creep into your claims and evidence and weaken the logic or your argument.

Fallacies are errors or tricks of reasoning. We call a fallacy an error of reasoning if it occurs accidentally; we call it a trick of reasoning if a speaker or writer uses it in order to deceive or manipulate his audience. Fallacies can be either formal or informal.

Most formal fallacies are errors of logic: the conclusion doesn’t really follow from, or is not supported by, the premises. Either the premises are untrue or the argument is invalid. For example, the following argument contains an error in assuming that one thing and only that one thing causes a particular outcome: “Nora feeds Johnny eggplant every day, and Johnny is really healthy; therefore, we should feed our Sally more eggplant.”  While it may be true that eggplant is a healthy food, Johnny’s eggplant consumption is assuredly not the only factor contributing to his health. Formal fallacies are created when the relationship between premises and conclusion does not hold up or when premises are unsound.

Informal fallacies take many forms and are widespread in everyday discourse. Very often they involve bringing irrelevant information into an argument, or are based on assumptions that, when examined, prove to be incorrect. For example, the following statement plays inaccurately to authority by assuming that one person only knows best, without bringing facts, multiple informed expert opinions, or relevant research into the argument: “Our governor supports a single-payer health care system for the state, so we should, too.”  While the governor may have information about health care that the average citizen does not have access to, just the fact that the governor supports a particular cause is not a reason to agree.  Informal fallacies often result from the misuse of language and/or evidence.

Whether a fallacy is an error or a trick, whether it is formal or informal, its use undercuts the validity and soundness of any argument. For example, if someone defines a key term in her argument in an ambiguous, vague, or circular way, her argument will appear very weak to a critical audience.

In addition, when listeners or readers spot questionable reasoning or unfair attempts at audience manipulation, more than their evaluation of the author’s argument (logos) may be compromised. Their evaluation of the credibility of the speaker (ethos), and perhaps their ability to connect with that speaker on the level of shared values (pathos), also may be compromised. At the very least, the presence of fallacies will suggest to an audience that the speaker or writer lacks argumentative skill.

The following video offers clear examples of some common logical fallacies.