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From Latin, fallacia: deliberate deceit or trick. Cf. Greek sophisma: skillful act, clever device, sly trick, specious argument.

A logical fallacy is, generally, an error or defect in reasoning (fallacious argument) that exhibits the following specific characteristics:

1. It is an incorrect, defective form of reasoning that differs from a merely factual error (false premises). In other words, the relation between the premises and conclusion is logically mistaken, i.e., the conclusion does not follow from the premises. The premises provide irrelevant evidence or insufficient evidence. Hence, the argument is invalid (deductive arguments) or weak (inductive arguments).

2. It is a common yet persuasive type of reasoning error that has proven over time its power to deceive people on a regular basis. As such, fallacies have been categorized and catalogued since Aristotle’s Sophistical Refutations, throughout the middle ages and modern period, and up to the present day. They are, therefore, not merely any mistaken inference, but a pervasive and persuasive class (general type, form, pattern, structure) of fallacious reasoning.

3. Accordingly, a fallacy is specious: it only appears or seems to be a good argument. However, when its initial plausibility is carefully examined it turns out to be an imposter, counterfeit, deceptive device, trick, trap, pretender, or sophism.

Arguments, like men, are often pretenders. —Plato (c.427-347 BCE)

It would be a very good thing if every trick could receive some short and obviously appropriate name, so that when anyone used this or that particular trick, he could at once be reproved for it.
                                                                      —Arthur Schopenhauer (1788-1860)

Before examining the types of fallacies, it is helpful to distinguish between the form and content of an argument. The form of an argument is its reasoning pattern or structure. In contrast, the content of an argument is its specific subject matter. Examples:

All humans are mortal                All politicians are saints
All logicians are human              All liberals are politicians
All logicians are mortal               All liberals are saints

These two arguments exhibit an identical form by means of, or through, their different content.

Identical Argument Form:

All As are Bs
All Cs are As
All Cs are Bs

The two arguments above substitute different content for the symbolic variables A, B, and C. As such, they are said to instantiate (be an instance of) a particular argument form. When a deductive argument instantiates a correct argument form it is called valid, i.e., the conclusion follows from the premises with strict necessity or, stated alternately, the conclusion must be true if the premises are true.

1. Formal Fallacies: The form of the argument is defective. Formal fallacies only apply to deductive arguments with identifiable reasoning patterns. Accordingly, a deductive argument that does not instantiate a correct (valid) argument form is said to contain or exhibit a formal fallacy. Two examples follow:

a) Affirming the Consequent: This fallacy is an argument of the form "A implies B, B is true, therefore A is true".

If a supernatural being created the universe, then we would see order and organization everywhere.
We do see order and organization everywhere.
Therefore, a supernatural being did create the universe.

If A then B
B is true
Therefore, A is true

b) Denying the Antecedent: This fallacy is an argument of the form "A implies B, A is false, therefore B is false".

If Myrth York is elected governor of RI, then RI will have a governor who supports same-sex marriages.
Myrth York will not be elected governor of RI.
Therefore, RI will not have a governor who supports same-sex marriages.

If A then B
A is false
Therefore, B is false

Both of these formal fallacies are deceptively persuasive. We often uncritically assume that if the hypothetical proposition (if A then B) is true, then the similar, yet logically distinct, hypothetical proposition (if B then A) must also true (Affirming the Consequent). For example, if there is smoke then there is fire, and if there is fire then there is smoke. But the hypothetical implication (if…then relation) is unidirectional (from A to B), not bidirectional (from A to B and from B to A). Also, we often uncritically assume that if the truth of A implies the truth of B, then it must also be the case that the falsity of A implies the falsity of B (Denying the Antecedent). But they are by no means logically equivalent.

2. Informal fallacies: The fallacy consist in the content and “can be detected only through analysis of the content of the argument" (Hurley, p.119). Classifications of informal fallacies differ, both historically and among contemporary logicians. For example, Aristotle identified only 13 fallacies and divided them into two categories: those related to language and those not related to language. In contrast, some contemporary logicians identify numerous (over 100) fallacies and divide them into several (5-10) classifications. Fallacies of relevance, presumption, and ambiguity are quite standard. We will follow Hurley’s classification, specifically fallacies of relevance and weak induction.


1. Deliberate Intent: Knowingly utilizing fallacious reasoning in order to persuade someone to believe or do X, where X will benefit the arguer(s). The arguer typically has an interest (political, economic, personal, psychological, etc.) in your accepting and/or acting upon their thesis. Examples:

  • politicians want your vote
  • dictators want to exploit your passions and prejudices
  • advertisers want you to purchase their product
  • scam or con artists want you to do X
  • fanatics or zealots (religious, political, nationalistic, etc.) want you to join them

2. Careless Intellectual Habits: Unknowingly and mistakenly believing that one’s reasoning is correct when in fact one is engaged in fallacious reasoning. The basis of such honest yet erroneous reasoning include:

  • mental laziness
  • emotional disposition
  • uncritical reasoning habits
  • one’s worldview (German: weltanshaunng)


1. Clarity: Consistently and persistently insist upon the exact meaning of the thesis to which you are being asked to assent (conclusion), as well as the exact meaning of each proposition offered as evidence for the thesis (premises).

2. Evidential Relevance:
Determine what type or kind of evidence is pertinent to establishing the thesis under consideration and regard with critical skepticism or suspicion any argument that does not provide appropriate evidence.

3. Unemotional Mental Maturity: Beware of any argument that evokes emotion of any kind, especially if you want to agree with the thesis/conclusion of a given argument. It is helpful in such contexts to restate the argument in the most unemotional way possible. The desire, wish, or need to believe X is a common and powerful obstacle to avoiding fallacies, whether committed by oneself or others.

4. Justified Suspicion: Be wary of arguments presented by anyone who might have a motive to deceive you. Generally, it is reasonable to be on your guard in social contexts in which you are exposed to individuals who stand to gain from your assent to their argument. Specifically, it is prudent to presume that some types of people might try to deceive you since they have a vested interest in having their thesis accepted: politicians want your vote, salespeople and telemarketers want you to buy something, a prosecutor or defense attorney want you to convict or acquit, religious fanatics or cultists want you to join their ranks, dogmatic ideologues of all stripes want you to tow the party line, scam or con artists want to “rip you off” (exploit you monetarily or in some other manner), etc. In general, anyone with a personal stake in persuading you of X warrants vigilant suspicion. This prudent logical practice should not, however, be confused with the informal fallacy of circumstantial ad hominem. In this fallacy someone’s argument is summarily dismissed out of hand, without any actual evaluation of their argument, simply because the arguer stands to gain from the acceptance of their argument. There is a considerable logical difference between being suspicious and being prejudicial.

5. Instinctive Sense: When you encounter a questionable argument that seems fishy to you, or doesn’t quite sound right, try to figure out why it seems erroneous to you, even if you can’t immediately identify what is specifically wrong with it. We may refer to this kind of hunch as the exercise of our instinctive or intuitive logical sense. Taking shape over time, whether consciously or not, this logical sensibility may be very advanced, very underdeveloped, or anywhere in between. You may ultimately conclude that the argument is reasonable or you may decipher the logical fallacy that prompted your initial doubts. What’s important is that you trust your instincts, at least initially and pending further reflections.

6. Recognizing Complexity: In contrast to formal fallacies, which instantiate one fallacious argument form, it is possible for an argument a) to contain more than one informal fallacy, or b) to be a border-line case that could be equally classified as one or another informal fallacy. In the context of everyday life (radio, TV, newspapers, political rhetoric, etc.) quite a few arguments contain more than one fallacy. What’s important is that you recognize that an argument is fallacious and then attempt to determine how (one or more ways) it is fallacious.