Logical Fallacies: Arguments, Reasoning, and the Fallacy

Fallacies are defects in an argument that cause it to be invalid, unsound, or weak. In a deductive argument, the existence of a fallacy means that the argument is not valid - even if the premises are true, the conclusion might still be false. A fallacy does not guarantee it is false; a fallacious argument fails to provide a good reason to believe the conclusion, even if that conclusion is correct.

Fallacies of Ambiguity: The No True Scotsman FallacyWhat is 'The 'No True Scotsman...' Fallacy and how is this fallacy committed? This is actually a combination of several fallacies, but since it rests ultimately on shifting the meaning of terms - a form of equivocation - and begging the question, it receives special attention.

Fallacies of Presumption: Begging the Question (Petitio Principii)This is the most basic and classic example of a Fallacy of Presumption, because it directly presumes the conclusion which is at question in the first place. This can also be known as a "Circular Argument" - because the conclusion essentially appears both at the beginning and the end of the argument, it creates an endless circle, never accomplishing anything of substance.

Fallacies of Relevance: Appeal to Authority Overview and IntroductionAppeal to Authority: A fundamental reason why the Appeal to Authority can be a fallacy is that a proposition can be well supported only by facts and logically valid inferences. But by using an authority, the argument is relying upon testimony, not facts. A testimony is not an argument and it is not a fact.

Appeal to Authority: Argumentum ad Populum (Appeal to Numbers)Appeal to Numbers This fallacy occurs any time the sheer numbers of people who agree to something is used as a reason to get you to agree to it and takes the general form: When most people agree on a claim about subject S, the claim is true (normally an unstated premise). Claim X is one which most people agree on. Therefore, X is true.

Fallacies of Relevance: Appeal to TraditionAppeal to Tradition: This form of the Appeal to Authority is slightly different from the others in that it does not directly make an appeal to the authority of any particular people. Instead, it makes its appeal to the authority of the collective interests and habits of people, as expressed in tradition or culture.

What is a Fallacy? Understanding Defective ArgumentsFallacies are defects in an argument that cause an argument to be invalid, unsound or weak. Fallacies can be separated into two general groups: formal and informal. A formal fallacy is a defect which can be identified merely be looking at the logical structure of an argument. Informal fallacies are defects which can be identified only through an analysis of the actual content of the argument.

Fallacies of Presumption: False Dilemma, Excluded Middle, False Dichotomy, BifurcationThe False Dilemma fallacy occurs when an argument offers a false range of choices and requires that you pick one of them. The range is false because there may be other, unstated choices which would only serve to undermine the original argument. If you concede to pick one of those choices, you accept the premise that those choices are indeed the only ones possible.

What is a Fallacy?Fallacies can be separated into two general groups: formal and informal. A formal fallacy is a defect which can be identified merely be looking at the logical structure of an argument rather than any specific statements.

Logical Fallacies: Alphabetical IndexAlphabetical list of all of the fallacies, formal and informal, which are cataloged and explained on this site. More are added regularly, so keep checking back over time.

Fallacies of Grammatical AnalogyArguments with this sort of defect have a structure which is grammaticaly close to arguments which are valid and make no fallacies. Because of this close similarity, a reader can be distracted into thinking that a bad argument is actually valid.

Fallacies of AmbiguityWith these fallacies, some sort of ambiguity is introduced either in the premises or in the conclusion itself. This way, an apparently false idea can be made to appear true so long as the reader does not notice the problematic definitions.

Fallacies of RelevanceFallacies in this category have in common the characteristic that they all make use of premises which are logically irrelevant to the final conclusion. However, this is not usually noticed because these same premises are often relevant for psychological reasons, thus making the conclusion seem to follow.

Fallacies of PresumptionFallacies of presumption are created when the premises of the argument presume something illegitimately. Sometimes what is presumed is the actual conclusion itself; however, it is also possible to presume some other important or related issues. In general, these are problems because if the assumptions were made explicit, they would provide grounds for questioning the conclusion.