Informal fallacies are a type of incorrect argument in natural language. The source of the error is not necessarily due to the form of the argument, as is the case for formal fallacies, but is due to its content and context. Fallacies, despite being incorrect, usually appear to be correct and thereby can seduce people into accepting and using them. These misleading appearances are often connected to various aspects of natural language, such as ambiguous or vague expressions, or the assumption of implicit premises instead of making them explicit.
Traditionally, a great number of informal fallacies have been identified, including the fallacy of equivocation, the fallacy of amphiboly, the fallacies of composition and division, the false dilemma, the fallacy of begging the question, the ad hominem fallacy and the appeal to ignorance. There is no general agreement as to how the various fallacies are to be grouped into categories. One approach sometimes found in the literature is to distinguish between fallacies of ambiguity, which have their root in ambiguous or vague language, fallacies of presumption, which involve false or unjustified premises, and fallacies of relevance, in which the premises are not relevant to the conclusion despite appearances otherwise.
Some approaches in contemporary philosophy consider additional factors besides content and context. As a result, some arguments traditionally viewed as informal fallacies are not considered fallacious from their perspective, or at least not in all cases. One such framework proposed is the dialogical approach, which conceives arguments as moves in a dialogue-game aimed at rationally persuading the other person. This game is governed by various rules. Fallacies are defined as violations of the dialogue rules impeding the progress of the dialogue. The epistemic approach constitutes another framework. Its core idea is that arguments play an epistemic role: they aim to expand our knowledge by providing a bridge from already justified beliefs to not yet justified beliefs. Fallacies are arguments that fall short of this goal by breaking a rule of epistemic justification. A particular form of the epistemic framework is the Bayesian approach, where the epistemic norms are given by the laws of probability, which our degrees of belief should track.
The study of fallacies aims at providing an account for evaluating and criticizing arguments. This involves both a descriptive account of what constitutes an argument and a normative account of which arguments are good or bad. Informal fallacies are expressed in natural language. This brings with it various difficulties not faced when studying formal fallacies, like ambiguous terms, vague expressions or the premises being assumed implicitly rather than stated explicitly. Traditionally, a great number of informal fallacies have been listed, including the fallacy of equivocation, the fallacy of amphiboly, the fallacies of composition and division, the false dilemma, the fallacy of begging the question, the ad hominem fallacy or the appeal to ignorance. The premises of an argument may be seen as the foundation on which the conclusion is built. According to this analogy, two things can go wrong and turn an argument into a fallacy. It could be that the foundation is shaky. But even a solid foundation is not helpful if it does not provide support for the conclusion in question.
The content of an argument is found on the level of its propositions: it is what is expressed in them. The source of many informal fallacies is found in a false premise. For example, a false dilemma is a fallacy based on a false disjunctive claim that oversimplifies reality by excluding viable alternatives. One way to explain that some fallacies do not seem to be deductively invalid is to hold that they contain various hidden assumptions, as is common for natural language arguments. The idea is that apparent informal fallacies can be turned into formal fallacies by making all these assumptions explicit and thereby revealing the deductive invalidity. The claim that this is possible for all fallacies is not generally accepted. Often many of the subtleties of natural language have to be ignored in this process. Some bodies of knowledge can be formalized without much residue but others resist formalization. This is also true for many informal fallacies.
Epistemic
The core idea behind the epistemic approach is that arguments play an epistemic role: they aim to expand our knowledge by providing a bridge from already justified beliefs to not yet justified beliefs. Whether an argument constitutes a fallacy or not depends on the credences of the person evaluating the argument. This means that what constitutes a fallacy for one arguer may be a sound argument for another. Such reformulations may include a condensation of the original argument in order to make it easier to spot the erroneous step.
: Therefore, feathers cannot be dark.
Equivocations are especially difficult to detect in cases where the two meanings are very closely related to each other.
Begging the question is a form of circular reasoning in which the conclusion is already assumed in the premises.
Ad hominem arguments constitute an important class among the fallacies of relevance. In them, the arguer tries to attack a thesis by attacking the person pronouncing this thesis instead of attacking the thesis itself. It is particularly associated with contemporary Russian propaganda.
Appeal to ignorance is another fallacy due to irrelevance. Arguments from analogy have the following form: a is similar to b and a has feature F, therefore b probably also has feature F. The soundness of such arguments depends on the relevance of this similarity to the inferred feature.