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Logical Fallacies, Long Lost Art of Rhetoric I Cheat Sheet by

Bad Arguments


A fallacy is the use of invalid or faulty reasoning.
Some fallacies are committed intent­ionally to manipulate or persuade by deception, while others are committed uninte­nti­onally due to carele­ssness or ignorance
Aristotle was the first to system­atize logical errors into a list, as being able to refute an opponent's thesis is one way of winning an argument
Richard Whately defines a fallacy broadly as, "any argument, or apparent argument, which professes to be decisive of the matter at hand, while in reality it is not"
The first principle is that you must not fool yourself and you are the easiest person to fool
Richard P. Feynman

Types of Fallacies

An error in logic in the argument's form.
Non Sequiturs
Propos­itional fallacies
Quanti­fic­ation fallacies
Syllog­istic fallacies
Reasons other than struct­ural, require examin­ation of the argument's content
Faulty genera­liz­ations
Red herring fallacies
Cond­itional or questi­onable
Arguments disregard or confusion
Other systems of classi­fic­ation
The most famous are those of Francis Bacon and J. S. Mill
Bacon divided fallacies into 4 Idola (Idols, False Appear­ances), summarize the kinds of mistakes the human intellect is prone.
Offend­icula of Roger Bacon
Opus maius,J. S. Mill book of his Logic,
Jeremy Bentham's Book of Fallacies (1824).
Whateley's Logic, A. de Morgan, Formal Logic (1847)
Sidgwick, Fallacies (1883)


Formal fallacies

Appeal to probab­ility
Takes something for granted because it would probably be the case Something can go wrong (premise). Therefore, something will go wrong (invalid conclu­sion)
Argument from fallacy
Aka fallacy fallacy, assumes that if an argument is fallac­ious, then the conclusion is false If P, then Q. P is a fallacious argument. Therefore, Q is false
Base rate fallacy
Making a probab­ility judgment based on condit­ional probab­ili­ties, without taking into account the effect of prior probab­ilities Police officers have breath­alyzers displaying false drunke­nness in 5% of the cases the driver is sober. However, the breath­alyzers never fail to detect a truly drunk person. One in a thousand drivers is driving drunk. The police officers stop a driver at random, and force the driver to take the test. The test is positive. You don't know anything else about him or her. How high is the probab­ility he or she really is drunk? Many would answer as high as 0.95, but the correct probab­ility is about 0.02. To find the correct answer, one should use Bayes's theorem
Conjun­ction fallacy
Assumption that an outcome simult­ane­ously satisfying multiple conditions is more probable than an outcome satisfying a single one of them Linda is 31 years old, single, outspoken, and very bright. She majored in philos­­ophy. As a student, she was deeply concerned with issues of discri­­mi­n­ation and social justice, and also partic­­ipated in anti-n­­uclear demons­­tr­a­t­ions. Which is more probable? Linda is a bank teller. Linda is a bank teller and active in the feminist movement. The majority of those asked chose second option. However the probab­ility of two events occurring together is always less than or equal to the probab­ility of either one occurring alone
Masked-man fallacy
Substi­tution of identical design­ators in a true statement can lead to a false one Lois Lane believes that Superman can fly. Lois Lane does not believe that Clark Kent can fly. Therefore Superman and Clark Kent are not the same person
Prop­osi­tional fallacies
A propos­itional fallacy is an error in logic that concerns compound propos­itions. For a compound propos­ition to be true, the truth values of its consti­tuent parts must satisfy the relevant logical connec­tives and, or, not, only if, if and only if
Affirming a disjunct
Concluding that one disjunct of a logical disjun­ction must be false because the other disjunct is true Max is a mammal or Max is a cat. Max is a mammal. Therefore, Max is not a cat
Affirming the consequent
The antecedent is claimed to be true because the consequent is true; if A, then B; B, therefore A If someone owns Fort Knox, then he is rich. Bill Gates is rich. Therefore, Bill Gates owns Fort Knox
Denying the antecedent
The consequent is claimed to be false because the antecedent is false; if A, then B; not A, therefore not B If you are a ski instru­ctor, then you have a job. You are not a ski instru­ctor, Therefore, you have no job
Quan­tif­ication fallacies
A quanti­fic­ation fallacy is an error in logic where the quanti­fiers of the premises are in contra­diction to the quantifier of the conclu­sion
Existe­ntial fallacy
An argument that has a universal premise and a particular conclusion Every unicorn definitely has a horn on its forehead

Informal Fallacies

Informal fallac­ies
Arguments that are fallacious for reasons other than structural (formal) flaws and usually require examin­ation of the argument's content.
Appeal to the stone
argum­entum ad lapidem Dismissing a claim as absurd without demons­trating proof for its absurdity A: Infectious diseases are caused by microbes B: What a ridiculous idea! A: How so? B: It's obviously ridicu­lous
Argument from ignorance
argum­entum ad ignora­ntiam It asserts that a propos­ition is true because it has not yet been proven false (or vice versa) There may be seventy kazillion other worlds, but not one is known to have the moral advanc­ement of the Earth, so we're still central to the Universe
Argument from incred­ulity
Appeal to common sense "I cannot imagine how this could be true; therefore, it must be false."­ Argument from ignorance
Argument from repetition
argum­entum ad nauseam, argumentum ad infinitum signifies that it has been discussed extens­ively until nobody cares to discuss it anymore; sometimes confused with proof by assertion
Argument from silence
argum­entum ex silentio conclusion is based on the absence of evidence, rather than the existence of evidence
Argument to moderation
ad temper­antiam false compro­mise, middle ground, fallacy of the mean. Assuming that the compromise between two positions is always correct
Argumentum verbosium
See: by verbosity
Begging the question
petitio principii providing what is essent­ially the conclusion of the argument as a premise Opium induces sleep because it has a soporific quality A kind of circular reasoning
Shifting the burden of proof
See: onus probandi I need not prove my claim, you must prove it is false
Circular reasoning
circulus in demons­trando when the reasoner begins with what he or she is trying to end up with; sometimes called assuming the conclusion Whatever is less dense than water will float, because such objects won't sink in water
Circular cause and conseq­uence
The conseq­uence of the phenomenon is claimed to be its root cause. Correl­ation does not imply causation
Continuum fallacy
Improperly rejecting a claim for being imprecise Fred is clean-­shaven now. If a person has no beard, one more day of growth will not cause them to have a beard. Therefore Fred can never grow a beard
Correl­ati­ve-­based fallacies
Corr­elation proves causat­ion post hoc ergo propter hoc a faulty assumption that because there is a correl­ation between two variables that one caused the other.
Supp­ressed correl­ative where a correl­ative is redefined so that one altern­ative is made impossible
Divine fallacy
Argument from incred­ulity. Because something is so incredible / amazing / ununde­rst­and­able, it must be the result of superior, divine, alien or paranormal agency
Double counting
Counting events or occurr­ences more than once in probab­ilistic reasoning, which leads to the sum of the probab­ilities of all cases exceeding unity
Misl­eading use of a term with more than one meaning
Ambiguous middle term a common ambiguity in syllogisms in which the middle term is equivocated
Defin­itional retreat changing the meaning of a word to deal with an objection raised against the original wording.

Draft Version

1 October 2020

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