In the world of philosophy and psychology, the concept of fallacy is very important, because it gives an idea of the quality of the reasoning that we can use to argue a point of view.

What is a fallacy? It is simply an error in reasoning, a type of argumentation in which the premises used do not lead to a conclusion. In fact, the term is derived from the word “fallare”, which means to lie or deceive. In other words, it serves to emphasize the deceptive character of these arguments.

But recognizing a fallacy is not easy, because it can take different forms. In fact , there are many types of fallacies, some of which are not at all similar to each other . It is important to know them well if one wants to ensure the quality of the debates and the processes of knowledge generation in a valid way. In the end, a “slip-up” can lead to a totally wrong conclusion.

Formal and informal fallacies

The most general classification that can be made of fallacies is that which distinguishes between formal fallacies and informal ones . While in the latter the reasoning error has to do with the content of the propositions, in formal fallacies the reasoning error is in the way the propositions are related to each other. Therefore, formal fallacies are always objective, while in the case of informal fallacies a debate may arise as to whether or not there is an argumentation error, given that their nature always depends on the context in which they are used.

For example, trying to discredit an idea by talking about negative aspects of the speaker is an ad hominem fallacy, but the same is not true if talking about the speaker gives relevant information that should be brought up. If the person who decides to focus the discussion on a worker’s misconduct is known to be trying to mob him or her In the case of formal fallacies, there is no room for discussion, in any case one can examine whether the concepts used are correct (for example, whether the same word has two different meanings throughout the logical operation).

In this article we will focus on analyzing the types of formal fallacies. To learn more about the types of fallacies in general, you can visit this article.

Types of formal fallacies and examples

Next we will review the main types of formal fallacies.

1. False disjunctive syllogism

This fallacy is based on a disjunction of style “A and/or B” . When one of the possibilities is stated, the other is assumed to be false. Of course, this conclusion is not derived from the premises.

Example : “You can eat or shower if you wish. You are taking a shower, so you are not going to eat. This fallacy is not such when the disjunction is exclusive: “or A or B.”

2. Affirmation of the consequent

In this formal fallacy it is assumed that if a premise is true , then the consequence of this premise also indicates whether its predecessor is true or not.

Example : “If I study hard I’ll get the highest grade, so if I get the highest grade I’ll have studied hard.”

3. Denial of background

In this kind of formal fallacy the reasoning is articulated as if by denying a premise the conclusion of the premise had to be necessarily false .

Example : “If it rains, the street will get wet; it hasn’t rained, so the street won’t get wet.”

4. False denial of the conjunction

This fallacy occurs when as there is no phenomenon as a result of a set of elements, one of those elements is denied .

Example : “To make a good cake you need flour and cream; there is no good cake left, so no cream.”

5. Undistributed medium term

In this fallacy there is an element that connects two others and that does not appear in the conclusion , although one of them is not completely covered.

Example : “All mammals have eyes, some molluscs have eyes, therefore some molluscs are mammals.

6. Categorical syllogism with negative premises

This fallacy occurs in any categorical syllogism in which both premises are a negation , since nothing can be concluded from them.

Example : “No mammal has feathers, no mouse has feathers, so no mammal is a mouse.”

7. Categorical syllogism with negative conclusion from affirmative premises

In categorical syllogisms a negative conclusion cannot be obtained from affirmative premises , and to do so is to fall into fallacious reasoning.

Example : “All Germans are European and some Christians are European, so Christians are not German.

8. Four-term fallacy

In this fallacy there are four terms, instead of three, which would be necessary for it to be valid . This is because one of the terms has two meanings.

Example : “Man is the only animal capable of taming fire, woman is not a man, so woman cannot tame fire.”