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An informal fallacy is a fault in an argument or statement deriving from the content of the statement rather than some breakdown in logic. There are dozens of types of errors that may result in an informal fallacy, but most can be classified as fallacies of relevance, presumption, or ambiguity. Each of these classifications cover several specific informal fallacies, and many others do not fit neatly into either category.
In this context, “informal” is not used to mean casual or imprecise. Instead, “formal” and “informal” identify the flawed portion of an argument. A formal fallacy contains a flaw in the form of the argument where logic is incorrectly applied. An informal fallacy is incorrect due to the content of the argument.
Fallacies of relevance attempt support arguments with irrelevant facts and statements. Facts provided may be true, but the information does not demonstrate that the premise being defended is true. Many of the most common fallacies are identified by Latin names.
For instance, an "Ad Hominem" fallacy attempts to prove an argument by discrediting the other party. Supporters of the opposing position may or may not be “mean”, “greedy”, or “lazy”. That doesn’t address whether the position they hold is valid or true, clearly identifying such slurs as informal fallacies.
Some fallacies of relevance attempt to sway opinion based on outside influences. An "Ad Verecundiam" fallacy claims support by some authority figure as proof that a statement is true. "Ad Populum" fallacies work in much the same way, with support coming from popular opinion rather than one individual or authority.
Presumption can also lead to an informal fallacy, confusing coincidence with cause and effect. Generalizations and circular reasoning are sometimes used to a similar effect. In a straw man argument, an opposing view is exaggerated, and this unreasonable exaggerated view is attacked.
Another common type is the fallacy of ambiguity. Words often have multiple meanings, sometimes with only subtle differences. When multiple meanings of a word or phrase are used to support an argument, or when meanings are unclear, a fallacy of ambiguity is the result.
There are dozens of forms of informal fallacy. Many are overt attempts to steer an argument away from reason. Others are quite subtle and may initially appear as reasonable arguments. Some of these approaches can be effective in debate and influencing opinions, but these are most often used because relevant facts and logic do not support the stated position.