Chapter 3: MAKING SENSE OF
ARGUMENTS
Exploring in more depth the nature of
arguments
 Evaluating them
 Diagramming them

ARGUMENT BASICS
Arguments allow us to support claims
and to evaluate claims
 2 Forms: Deductive and Inductive
 Deductive: to deduce means to draw
out or distill
 Intended to provide CONCLUSIVE
support

ARGUMENTS
Inductive: to broaden out.
 Intended to provide PROBABLE
support

More on Deductive Arguments
Validity: if premises are true, then
conclusion must be true.
 Guaranteed conclusion (All or
nothing)
 Necessity
 Truth Preserving: The conclusion
cannot be false if the premises are true.

Examples: Deductive
Socrates is a man.
All men are mortal.
Therefore, Socrates is mortal
 Example in invalid argument with
same form:
 All dogs are mammals.
All cows are mammals.
Therefore, all dogs are cows

Examples: Deductive
If Socrates is a man, then he is mortal.
Socrates is a man.
Therefore, Socrates is mortal

Invalid form:
 If Socrates has horns, he is mortal.
He is mortal.
Therefore he has horns.
INDUCTIVE ARGUMENTS
probable logical support
 Strong and Weak
 Structure of Inductive Arguments
cannot guarantee that if the premises
are true the conclusion must also be
true.
 Implies: premises can be true, and
conclusion still questionable.

Slippage/free play:
Conclusion always goes a bit beyond
what is contained in premises.
 The idea of Gap:
 It is always possible to go to another
conclusion, sometimes even an
opposite one with weak arguments.

Degrees of Strength
varying from weak, to modestly weak,
to modestly strong and to strong

eg. Most dogs have fleas
My dog Bowser, therefore, probably
has fleas.
What about the premise here?

SOUNDNESS:
Applied to deductive arguments.
When arguments have true premises
and true conclusions (to be sure).
 It is possible to have valid deductive
arguments while having false
premises and false conclusions.
 Page 69-70 in text

COGENCY
applies to inductive arguments
 When inductive arguments have true
premises
 Good inductive arguments are both
strong and cogent

JUDGING AND EVALUATING
ARGUMENTS

Skills to start
1. identifying form: inductive or
deductive
 Mixed Arguments
 2. Determining or judging whether it is
cogent or sound

A STRATEGY: 4 Steps
1. Identify conclusion and premises.
Even number them.
 2. Test of deduction: Do the premises
seem to make the conclusion
necessary? LOOK TO FORM!
 3. Test of Induction: What degree of
probability do the premises confer on
the conclusion?

STRATEGY Cont.
Are the premises true (cogency)? If
no, go to 4.
 4. Test of Invalidity and weakness:
Only 2 options left.


Does the argument intend to offer
conclusive or probable support but
fail to do so?
Form and Indicator words
Some examples from text pp. 74-75
and Exercise 3.2

FINDING MISSING OR IMPLIES
PREMISES
What are they? Premises essential to
the argument that are left unstated or
unspoken
 i.e. Socrates in the deductive
argument
 Assumes there was someone named
Socrates, etc.

Implied Premises, con.
Text: P. 79
 “Handguns are rare in Canada, but
the availability of shotguns and rifles
poses a risk of death and injury.
Shotguns and rifles should be
banned, too!”
 Implied premise: Anything or most
anything that poses a risk of death or
injury should be banned.

IMPLIED PREMISES cont.
The Point: We need to evaluate also
this implied premise.
 Other examples. Page 80.

SOME IMPORTANT HINTS
1. It is best always to identify missing
premises. We cannot take them for
granted.
 2. Formulate the implied premise with
as much charity as possible.
 3. Premise should be plausible (or, as
strong as possible)

IMPLIED PREMISES, cont.

4. Premise fits author’s intent

5. Principle of connecting
unconnected terms
FULL EVALUATION:
Degree of controversy of both given
premises and implied premises.
 What further support do they require?
 P. 81-82 example
 Exercise 3.4 (I: 1, 3, 6, 9)

ARGUMENT PATTERNS
Hypothetical syllogism
 E.g.
If the job is worth doing, then it’s
worth doing well.
The job is worth doing.
Therefore, it is worth doing well.

ARGUMENT PATTERNS

2 Patterns to start:








1. Hypothetical
2. Disjunctive
3. Categorical
Hypothetical has two parts
Antecedent: the job is worth doing
Consequent: the job is worth doing well.
Antecedent: p
Consequent: q
FORMS
Form: Modus Ponens and valid:
 Affirming the antecedent.
if p, then q
p.
therefore , q

FORMS
Another valid Form: Modus Tollens
E.G:
If Austin is happy, then Barb is happy
Barb is not happy.
Therefore, Austin is not happy.
Denying the consequent!
Pure Hypothetical Syllogism
if p, then q
if q, then r
if p, then r
Pure Hypothetical Syllogism:
If polar bears thrive, then they eat more
seals.
If they eat more seals, they will gain
more weight.
Therefore,
If polar bears thrive, they will gain more
weight.
INVALID FORMS
eg. If Dogbert commits one more
fallacy, I will eat my hat.
Dogbert did not commit one more
fallacy.
Therefore, I did not eat my hat.
p. 89 in Review Notes

DISJUNCTIVE SYLLOGISMS

eg. Either O.J. will go to jail, or his
lawyer will do a good job to get him
off.
O.J. did not go to jail.
Therefore, his lawyer did a good job to
get him off.
 FORM:
 either p or q
not p
q
DISJUNCTIVE SYLLOGISMS

Disjuncts:
P= O.J. will go to jail
 Q= His lawyer will do a good job ….

DIAGRAMMING ARGUMENTS
1. Underline indicator words, if
present
 2. Number all statements (or
propositions) in sequential order.
 3. Break down compound statements
(statements using connectives ‘and,’
‘but,’ ‘or’) into single statements.

DIAGRAMMING ARGUMENTS
Caution sometimes ‘or’ should not be
broken down.
 4. Cross out extraneous or irrelevant
statements. None-premises or
conclusions. Preludes, redundant
statements, or background
information.

DIAGRAMMING, cont.

Page 93 and on.
Pulling it all together

1. Diagram argument

Implies identifying premises, conclusions, etc.

2. Determine type based on form
 3. Evaluate:



For deductive determine whether valid or not,
sound or not
For non-deductive, determine degree of
strength and cogency
Borderline cases: mixed forms
Pulling it all together, cont.

Full Evaluation of Non-deductive
Measure gap between premises and
conclusion
 Identify implied premises and judge truth
 Ask whether other premises need to be
added to support implied and explicit
premises
 Determine whether we can get from
given premises to other or opposite
conclusions
