Chapter 4: Fallacies: When Arguments turn bad
Three categories of fallacies:
Logical fallacies: Arguments with irrelevant conclusions
Types of logical fallacies with conditional arguments:
Affirming the Consequent:
• If P then Q
• Therefore P
Denying the Antecedent:
• If P then Q
• It is not the case that P
• Therefore, it is not the case that Q
Tells us the truth of the antecedent is enough for the truth of the consequent, but the
antecedent does not necessarily lead to the truth of the consequent.
• Misordering of a universal quantifier (all, every, each) and an existential
quantifier (some,a,the,one) resulting in a invalid inference.
• Examples: 1. Every(u.q) person shops for clothes at one(e.q) store in town.
2. There is one (e.q) store in town which every (u.q) person shops for
A scope fallacy is committed when the universal quantifier moves to the existential
Fallacy of Equivocation:
• To argue by switching meaning. It disguises an invalid inference by using a single
expression in two different ways. • Example: the world expect; which could mean encourage, or predict
Evidential Fallacies :
• Are deductively invalid but also inductively unreliable.
• A good evidential argument shows its conclusion to be reasonably likely- with all
the vagueness and context-dependence that reasonably likely suggests.
Types of Evidential Fallacies:
Argument from Ignorance (Argument from Lack of Evidence):
• We have no evidence that P,
• It is not the case that P.
Just because something is true doesn`t mean that we can assert as being false, like
saying something doesn`t exist because there is no information on it.
A.I is always a logical fallacy, but that is not its interest.
Not the argument from Ignorance:
• We have excellent evidence that P.
This argument is invalid because the premise can be true while the conclusion is false.
It shows the conclusion to be reasonable in light of the information available. However
the truth of the claim does not validly follow from the fact that we have excellent
evidence in its favour.
An argument from lack of evidence is reasonable when it can correctly be framed in the
form of Modus Tollen argument:
1 Evidential reasoning is to detect and characterize correlations between objects, trends, and
abstract phenomena. • If P were true, then we should expect to find evidence that P is investigative
• Using investigative means M, we have been unable to find evidence that P.
• There are good grounds to regard P as untrue.
The truth is (1) is crucial—requiring us to have reason to regard M as an appropriate
means of revealing whether P is true.
What distinguishes evidential fallacies from logical fallacies is their failure to provide to
good evidence for their conclusions. Evidential fallacies are patterns of reasoning that
are often appealing, despite having weak or absent evidential connections between
premises and conclusions. They are much more frequently committed than logical
Hasty generalizations or sweeping generalizations consists of drawing a general
inference too strong for the specific evidence in hand.
A planned cooperative effort, usually to do with something illegal and usually with the
aim of keeping it quiet,
Argument of conspiracy:
• There is no evidence that P
• No evidence is exactly what we should expect, if P is true.
Argument of Authority or Genetic fallacy:
Evaluating a claim on the basis of irrelevant facts about its origins, rather than on the
basis of the evidence for it, the justification may be evidentially cogent. The argument is
based on the fact that so and so said so, usually someone famous or someone with
expertise in a given field.
Fallacy of appeal to vicarious authority: • Professor X said that P.
Standards for evaluating expert opinion:
• Relevant expertise
• Recent expertise
• Reason to believe that the opinion flow from the expert knowledge rather than
from other commitments or motives
• Degree of consistency with broader expert opinion
Knowing enough to evaluate expert opinion by these standards requires you to learn
something about the field, that is, independently of believing the specific opinion in
Appeal to popular opinion:
• It is widely believed that P
Fallacy of False Dichotomy: either the popular belief is correct, or the people who hold it
Argument from majority opinion among experts:
• Inference from “most relevantly defined experts say that P” to “It is true that p” to
is logically invalid.
• As evidential argument, this one is much stronger than the case of a single
authority very much stronger than the case of an irrelevant authority, and vastly
stronger than the case of mere popular opinion.
• It’s prima facie (first glance) rational to believe what the majority of experts in a
• Always defensible.
Post Hoc Ergo Propter Hoc, or Magical thinkin