Informal fallacy

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. In philosophy, fallacies are usually seen as a form of bad argument and are discussed as such in this article. Another conception, more common in non-scholarly discourse, sees fallacies not as arguments but rather as false yet popular beliefs.

Informal fallacies are a form of incorrect argument in natural language. An argument is a series of propositions, called the premises, together with one more proposition, called the conclusion. The premises in correct arguments offer either deductive or defeasible support for the conclusion. The source of the error in incorrect arguments can be in the argument's form, content or context. If the error is only due to the form, it is considered a formal fallacy. Informal fallacies may also include formal errors but they primarily involve errors on the level of content and context. 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 traditional approach tries to account for these fallacies using the concepts and theses discussed in this section.

Only arguments can constitute a fallacy. Various erroneous expressions do not count as fallacies because no argument is made, e.g. because no reasons are cited or no assertion is made. The core idea of arguments is that the premises support the conclusion or that the conclusion follows from the premises. Deductively valid arguments offer the strongest form of support: for them, it is impossible for the conclusion to be false if all the premises are true. The premises in non-deductive arguments offer a certain degree of support for their conclusion but they are defeasible: it is possible for all the premises to be true and the conclusion to be false. Defeasible arguments may still be rationally compelling despite being fallible, so they do not automatically constitute fallacies. 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.

Traditionally, fallacies have been defined by three necessary conditions: "a fallacy (i) is an argument, (ii) that is invalid, and (iii) appears to be valid." This definition covers only formal fallacy since it has deductive invalidity as a necessary condition. But it can easily be modified to include informal fallacy by replacing this condition with a more general term, like logical weakness or incorrect reasoning. The last clause includes a psychological element in referring to how the argument appears to the arguer. This clause is used to distinguish genuine fallacies from mere mistakes in reasoning, for example, due to carelessness. The idea is that fallacies have an alluring element that goes beyond mere carelessness by seducing us into committing the mistake, thereby explaining why they are committed in the first place. Some philosophers reject this appeal to appearances because the reference to psychology would complicate the investigation in various ways. One issue is that appearances are different for different people. This problem also involves social studies in order to determine which reference group of people to consult for defining fallacies. It has been suggested that, at its core, the study of fallacies is about normative aspects of arguments and not about their persuasive force, which is studied by empirical psychology instead.

The source of the error in incorrect arguments can lie in the argument's form, content, or context. The form or structure of an argument is also called "rule of inference". The most well-known rule of inference is modus ponens, which states that given a premise of the form "If p then q" and another in the form "p", then the conclusion is "q". Rules of inferences are formal because it depends only on the structure or the syntax of the premises and not on their content. So an argument based on modus ponens is valid no matter what propositional contents are used for "p" and "q".