An Illustrated Book of Bad Arguments
Logical Fallacy
An error in reasoning that results in an invalid argument. Errors are
strictly to do with the reasoning used to transition from one
proposition to the next, rather than with the facts. Put differently,
an invalid argument for an issue does not necessarily mean that the
issue is unreasonable. Logical fallacies are violations of one or more
of the principles that make a good argument such as good structure,
consistency, clarity, order, relevance and completeness.
Informal Fallacy
Red Herring
Summary
Ambiguity
Summary
Unwarranted Assumption
Summary
Casual Fallacy
Summary
Weak Analogy
Summary
Fallacy of Missing Data
Summary
Not a Cause for a Cause
Summary
Begging the Question Fallacy
Summary
Formal Fallacy
Propositional Fallacy
Summary
Logical Fallacy
Informal Fallacy
A logical fallacy that is due to its content and context rather than its
form. The error in reasoning ought to be a commonly invoked one for
the argument to be considered an informal fallacy.
Formal Fallacy
A logical fallacy whose form does not conform to the grammar and rules of inference of a logical calculus. The argument's validity can be determined just by analyzing its abstract structure without needing to evaluate its content.
Propositional Fallacy
Affirming the Consequent
Affirming the Consequent
One of several valid forms of argument is known as modus ponens (the mode of affirming by affirming) and takes the following form: If A then C, A; hence C. More formally:
A ⇒ C, A ⊢ C.
Here, we have three propositions: two premisses
and a conclusion. A is called the antecedent and C the consequent. For
example, If water is boiling at sea level, then its temperature is
at least 100°C. This glass of water is boiling at sea level; hence its
temperature is at least 100°C. Such an argument is valid in addition to being sound.
Affirming the consequent is a formal fallacy that takes the following form: If A then C, C; hence A.
The error it makes is in assuming that if the
consequent is true, then the antecedent must also be true, which in
reality need not be the case. For example, People who go to university are more successful in life. John is successful; hence he must have gone to university.
Clearly, John's success could be a result of schooling, but it could
also be a result of his upbringing, or perhaps his eagerness to overcome
difficult circumstances. More generally, one cannot say that because
schooling implies success, that if one is successful, then one must have
received schooling.
Red Herring
Argument From Consequences
Straw Man
Summary
Unwarranted Assumption
Content
Fallacy of Missing Data
Content
Not a Cause for a Cause
Content
Begging the Question Fallacy
Content
Argument From Consequences
Arguing from consequences is speaking for or against the truth of a
statement by appealing to the consequences of accepting or rejecting it.
Just because a proposition leads to some unfavorable result does not
mean that it is false. Similarly, just because a proposition has good
consequences does not all of a sudden make it true. As David Hackett
Fischer puts it, “it does not follow, that a quality which attaches to
an effect is transferable to the cause.”
In the case of good consequences, an argument may
appeal to an audience's hopes, which at times take the form of wishful
thinking. In the case of bad consequences, such an argument may instead
appeal to an audience's fears. For example, take Dostoevsky's line,
“If God does not exist, then everything is permitted.” Discussions of
objective morality aside, the appeal to the apparent grim consequences
of a purely materialistic world says nothing about whether or not the
antecedent is true.
One should keep in mind that such arguments are
fallacious only when they deal with propositions with objective truth
values, and not when they deal with decisions or policies [Curtis], such
as a politician opposing the raising of taxes for fear that it will
adversely impact the lives of constituents, for example.