Workshop on Logic PowerPoints

Lawyers, Logic, and
Logical Fallacies
Profs. Streitz & Crouse
Deductive Logic
• Valid:
if the
premises are true
then the conclusion
is necessarily true
• Sound:
if the form
is valid and the
premises are in fact
Modus Ponens
“the way or mode of positing”
Modus Ponens
• 1. If A, then B
2. A.
3. B
• Example:
1. If Lassie is a
Collie, then Lassie is a
2. Lassie is a Collie
So, 3. Lassie is a dog.
Modus Ponens
• Lines 1 and 2 are the
premises, and line 3 is
the conclusion.
• Line 1 is the major
• Line 2 the minor premise.
Modus Ponens
• For our purposes, the
major premise is usually
a rule of law (cReac)
• The minor premise is a
factual claim (creAc)
Hypothetical 1
4.Justifiable (Reliance)
• If O then ~J
• O
• ~J
• If SO then J
• SO
• J
Formal Fallacy:
Affirming the Consequent
• 1. If A, then B
2. B
3. A
• While the premises
may both be true,
they do not guarantee
the conclusion.
Formal Fallacy:
Affirming the Consequent
• 1. If SO, then J
2. J
3. SO
Modus Tollens
“the way that denies by denying”
Modus Tollens
• 1. If A, then B.
2. Not B.
3. Not A.
• Example:
1. If it is snowing, then
the ground is white.
2. The ground is not white.
So, 3. It is not snowing.
Hypothetical 2
1.Defamatory Statement
2.About the Plaintiff
3.Verbal Publication
• If D then PL
• ~PL
• ~D
• If SPS then D
• D
Formal Fallacy:
Denying the Antecedent
• 1. If A, then B
2. Not A
3. Not B
• While the premises
may both be true,
they do not guarantee
the conclusion.
Formal Fallacy:
Denying the Antecedent
• 1. If D, then PL
2. ~D
3. ~PL
Disjunctive Syllogism
Disjunctive Syllogism
• Disjunctions are statements of
the form "Either A or B". The
parts of a disjunction are
called disjuncts.
Disjunctive Syllogism
• 1. Either A or B
2. Not A.
So, 3. B
• Example:
Either Atlanta is a state or
Atlanta is a city.
Atlanta is not a state.
So, Atlanta is a city.
Hypothetical Syllogism
Hypothetical Syllogism
• 1. If A, then B.
2. If B, then C.
3. If A, then C.
• Hypothetical syllogism is one of
the proof rules in classical
logic, and is a valid argument
form. This kind of argument states
that if one implies another, and
that other implies a third, then
the first implies the third.
Hypothetical Syllogism
• The argument form is called
hypothetical syllogism because it
involves only hypothetical (if-then)
statements. Every argument that
exemplifies this form is a valid one.
• Note that contrary to many
definitions of deductive reasoning,
this is not reasoning from the
general to the particular.
• In legal reasoning, this form is
often coupled with modus ponens to
create a more complex argument.
Hypothetical Syllogism
• Example:
• If defendant’s counsel sleeps for a long
time at trial, then counsel has slept
through a significant portion of trial
• If counsel has slept through a
significant portion of trial,
defendant’s 6th Amendment right to
counsel has been violated
• Therefore, if defendant’s counsel sleeps
for a long time at trial, defendant’s 6th
Amendment right to counsel has been
Informal or Non-Deductive
– just kidding
• A claim that two or more things
that are similar to each other
in some respect(s) are also
similar (or should be treated
similarly) in other respects.
• In a legal argument, an analogy
may be used when there is no
precedent on point.
• Reasoning by analogy often
involves referring to another
case that concerns similar
facts and legal principles.
• A statistical syllogism is an example
of inductive reasoning:
• 90% of humans are right-handed.
• Joe is a human.
• Therefore, the probability that Joe is
right-handed is 90%.
• A stronger example:
• 100% of life forms that we know of
depend on liquid water to exist.
Therefore, if we discover a new life
form it will probably depend on liquid
water to exist.
• Inductive Arguments in Law?
• Reasoning to the best
• What detectives do
(and Sherlock Holmes
mislabeled as “deductive”).
• Abductive reasoning in law?
• Cumulation of consideration or
balance of consideration arguments.
• Example: (1) I will take the job in
Chicago, because (2) people are nice
there, (3) the pay is good, (4) the
infrastructure is good there, (5) I
have friends there, (6) the working
hours are reasonable, and (7) the
public transport system is great. But
(8) beer is overpriced.
• Conductive reasoning in Law?
• Balancing Tests
• Example: FRE 403 allows the
court to exclude relevant
evidence if its "probative
value is substantially
outweighed by the danger of
unfair prejudice, confusion of
the issues, misleading the
jury, or by considerations of
undue delay, waste of time, or
needless presentation of
cumulative evidence.”
Informal Fallacies
What are fallacies?
• Fallacies are defects that weaken arguments.
• By learning to spot them, you can strengthen
your ability to evaluate the arguments you
make, read, and hear.
• It is important to realize two things about
• First, fallacious arguments are very common and
can be persuasive, at least to the casual
reader or listener. Great for juries, but not
appellate judges (or legal writing
• Second, it is sometimes hard to evaluate
whether an argument is fallacious.
Fallaciousness (is that a word? fallaciosity?)
comes in degrees, and may depend on context.
Appeal to Authority
• Often we add strength to our
arguments by referring to respected
sources or authorities and explaining
their positions on the issues we're
discussing. If, however, we try to
get readers to agree with us simply
by impressing them with a famous name
or by appealing to a supposed
authority who really isn't much of an
expert, we commit the fallacy of
appeal to authority.
Appeal to Authority
• To philosophers or theologians:
a fallacy, the seriousness of
which is measured by the
reliability of the authority.
An appeal to anyone less than
God or anything less than the
Platonic forms of truth or
beauty is suspect.
Appeal to Authority
• To lawyers: a slam-dunk, inyour-face, nanny-nanny-boo-boo,
I-win-you-lose argument. The
gold standard. As long as it is
binding authority.
Appeal to Pity
• Definition: The appeal to pity
takes place when an arguer
tries to get people to accept a
conclusion by making them feel
sorry for someone.
Appeal to Pity
• Who is your audience?
– Philosophers: don’t do it
– Judges: you’d better be subtle
– Juries: have at it?
Hasty Generalization
• Definition: Making assumptions
about a whole group or range of
cases based on a sample that is
inadequate (usually because it
is atypical or too small).
• Stereotypes about people or
entities (e.g., “corporations
put profits over people”).
Hasty Generalization
• Example: I have seen 208
shirtless men in the last year.
203 of them committed felonies.
The vast majority of men
without shirts are felons.
Hasty Generalization
• Problem: My sample was 43
reruns of the TV show Cops.
Probably an atypical sampling
of American males.
• I still think my conclusion is
Begging the Question
• A complicated fallacy; it comes in several
forms and can be harder to detect than
many of the other fallacies we've
• Basically, an argument that begs the
question asks the reader to simply accept
the conclusion without providing real
• The argument either relies on a premise
that says the same thing as the conclusion
(which you might hear referred to as
"being circular" or "circular reasoning"),
or simply ignores an important (but
questionable) assumption that the argument
rests on.
Begging the question
“The defendant’s
conduct was reckless
because he recklessly
caused the accident by
driving too fast.”
Begging the question
• Pet peeve: misusing the phrase
“begs the question.”
• Please, don’t ever say that
something that raises a
question “begs the question.”