Logical Fallacies

Social ProblemsAngie Andriot

What is an argument?

A premise used to establish a conclusion• Facts to support a case• Objective versus subjective conditions

DEDUCTIVE arguments• General to specific• Laws, rules, principles

INDUCTIVE arguments• Specific to general• Experience, observation

Let’s apply this to Gender…

All the girls I know like pink

Pink is a girl color

Jill will like pinkJack won’t like pink

What are logical fallacies?

A fallacy is an error of reasoning• This differs from a factual error • The premise does not establish the conclusion

Formal (deductive) Fallacies • Valid: All x are y, z is x, therefore z is y• Invalid: All x are y, z is y, therefore z is x

Informal (inductive) Fallacies• Strong: Every documented x is y, therefore all

x are y• Weak: Every x I’ve ever seen is y, therefore all

x are y

X = girls, y = like pink/don’t like pink, z = specific girl or boy

Types of Fallacies

There are lots of different ways to break the fallacies down into subtypes

I’m going to go with a method that makes sense to me

It’s best represented by this website, if you’re interested in further info

Fallacies of Distraction, or “Look! A Monkey”

Illegitimate use of a logical operator• The Complex Question

Conjunction (“and”)• The False Dilemma

Disjunction (“or”)• The Slippery Slope

Conditional (“if-then”)• The Argument from Ignorance

Negation (“not”)

The Complex Question

A loaded question - the writer joins two issues together and asks the reader to treat them as one. • Have you stopped beating your wife? • Do you support freedom and the right to

bear arms? • “Pro-life” versus “Pro-choice”

Identify the two propositions illegitimately conjoined and show that believing one does not mean that you have to believe the other.

The False Dilemma

Two options are given while a third is ignored • Constraining answer choices• “Black and White” reasoning

You are either with me or against me• Actually, I couldn’t care less either way

Religion is necessary because otherwise there’d be no morality

Identify the options given and show (with an example) that there is an additional option.

The Slippery Slope

If this is allowed to happen, a series of increasingly unacceptable events will result.  So we shouldn’t do it.(1) If we allow gays to marry, next thing

you know we’ll be allowing people to marry their sisters, children, and pets

(2) People should not be allowed to marry their pets

Therefore:(3) Gays shouldn’t be allowed to marry

Identify the proposition being refuted and identify the final event in the series of events. Then show that this final event need not occur.

Argument from Ignorance

We haven’t proven it to be false, therefore it must be true• Or conversely, we haven’t proven it to

be true, therefore it must be false Falsifiability, burden of proof

• Russel’s teapot, invisible pink unicorns The absence of proof does not prove

anything• You have no alibi, clearly you did it!

Identify the proposition in question. Argue that it may be true/false even though we don't know whether it is or isn't.

Inductive Fallacies Inductive reasoning is not, in and of itself,

fallacious Good inductions are ones in which the

sample is similar to the population, and for which the support is strong

Inductive Fallacies include:• Hasty Generalizations• Unrepresentative Sample• False Analogies• Slothful Induction• Fallacy of Exclusion

Hasty Generalizations

Drawing a general rule from too small a sample or a single, perhaps atypical, case • Jews are manipulative. I know; my ex is

one. • I asked six of my friends what they

thought of the new spending restraints and they agreed it is a good idea.

Identify the size of the sample and the size of the population, then show that the sample size is too small. Note: a formal proof would require a mathematical calculation. This is the subject of probability theory. For

now, you must rely on common sense.

Unrepresentative Sample

The sample used in an inductive inference is relevantly different from the population as a whole. • The apples on the top of the box look

good. The entire box of apples must be good.

• I rarely encounter racism, so racism is rare

Show how the sample is relevantly different from the population as a whole, then show that because the sample is different, the conclusion is

probably different.

False Analogy (1) A and B are similar.

(2) A has a certain characteristic.Therefore:(3) B must have that characteristic too.

William Paley’s argument from design suggests that a watch and the universe are similar (both display order and complexity), and therefore infers from the fact that watches are the product of intelligent design that the universe must be a product of intelligent design too.

Identify the two objects or events being compared and the property which both are said to possess. Show that the two objects are different in a way

which will affect whether they both have that property.

Exclusion and Sloth

Slothful induction – the evidence doesn’t fit• Hugo has had twelve accidents n the last six

months, yet he insists that it is just a coincidence and not his fault.

Fallacy of Exclusion –unsupporting evidence is ignored or overlooked• The Leafs will probably win this game because

they've won nine out of their last ten. (Eight of the Leafs' wins came over last place teams, and today they are playing the first place team.)

Causal Fallacies

In general, we say that a cause C is the cause of an effect E if and only if:

• Generally, if C occurs, then E will occur, and • Generally, if C does not occur, then E will not occur

ether.

We say "generally" because there are always exceptions. For example, we say that striking the match causes the match to light, because:

• Generally, when the match is struck, it lights (except when the match is dunked in water), and

• Generally, when the match is not struck, it does not light (except when it is lit with a blowtorch).

Post HocMere temporal succession does not entail

causal succession: Immigration to Alberta from Ontario

increased. Soon after, the welfare rolls increased. Therefore, the increased immigration caused the increased welfare rolls.

(1) Most people who are read the last rites die shortly afterwards.Therefore:(2) Priests are going around killing people with magic words!

Show that the correlation is coincidental by showing that: (i) the effect would have occurred even if the cause did not occur, or (ii) that the effect

was caused by something other than the suggested cause.

Joint EffectOne thing is held to cause another when in fact both are the effect of a single underlying cause.

• Sleeping with one's shoes on is strongly correlated with waking up with a headache.

• Therefore, sleeping with one's shoes on causes headache.

Identify the two effects and show that they are caused by the same underlying cause. It is necessary to describe the underlying cause and

prove that it causes each symptom.

Some other causal errors

Wrong direction: “cancer causes smoking”• Increased weight leads to bad health?

Insignificance: By leaving your oven on overnight you are contributing to global warming.

Complex cause: The accident was caused by the poor location of the bush. (True, but it wouldn't have occurred had the driver not been drunk and the pedestrian not been jaywalking.)