Transcript 10. Critical Thinking Review

Critical Thinking
Review
Arguments
• An argument is always composed of
at least one premise and exactly
one conclusion.
• The premises provide (or at least are
claimed to provide) the conclusion
with:
– Support
– Evidence
– Justification
• The objective of critical thinking is to
determine just how good that
support is.
What is an Argument?
• An argument consists of a set
of premises followed by a
conclusion
• A premise is a statement that is
either true or false
• Likewise, a conclusion is also a
statement that is either true or
false
Two Types of Arguments:
Inductive & Deductive
• Good Inductive Argument:
– An argument in which the premises
provide good reasons for believing
the conclusion. In an inductive
argument, the premises make the
conclusion likely, but the
conclusion might be false even if
the premises are true.
• Valid Deductive Argument:
– An argument in which, without fail,
if the premises are true, the
conclusion will also be true.
Inductive & Deductive Arguments:
An Understanding
•We tend to call an argument
“Deductive” when we claim, or
suggest, or just hope that it is
deductively valid.
•And we tend to call an argument
“Inductive” when we want to
acknowledge that it is not
deductively valid but want its
premises to aspire to making the
conclusion likely.
Logic Definitions
•Premise: a statement which can be either
“true” or “false”
•argument: one or more premises which are
logically connected followed by a conclusion
•conclusion: the result that follows the
application of the laws of logic to the premises of
an argument
•tautology: an argument that is always true
Example: Either the sun shines or it does not
•contradiction: an argument that is always false
Example: I can fly and I cannot fly
Valid Arguments
• A valid argument is one in which the premises all
lead necessarily to the conclusion.
• An invalid argument is an argument in which the
premises either do not deductively lead to the
conclusion or where they are all true but the
conclusion is false.
• One key method of validating arguments is the
use of Ven diagrams.
Note: an argument’s content is irrelevant to its validity; it is the form that
determines validity. Of course arguments in which there is no logical connection
between premises and conclusion are just that ---- illogical and unfounded.
Valid and Invalid Arguments
• INVALID ARGUMENT
– Put in false premise(s)
– Put in true premise(s)
• VALID ARGUMENT
– Put in false premise(s)
– Put in true premise(s)
get out either true
or false conclusion
get out either true
or false conclusion
get out either true
or false conclusion
get out only true
conclusion
Valid Arguments and Ven
Diagrams
1. All blocks of cheese are more intelligent than
any philosophy student.
2. Meg the cat is a block of cheese.
3. Therefore, Meg the cat is more intelligent than
any philosophy student.
Blocks of cheese
Things that are more
intelligent than any
philosophy student
Meg the cat
Valid Arguments and Ven
Diagrams
1. All blocks of cheese are more intelligent than
any philosophy student.
2. Meg the cat is a block of cheese.
3. Therefore, Meg the cat is more intelligent than
any philosophy student.
Blocks of cheese
Things that are more
intelligent than any
philosophy student
Meg the cat
Valid Arguments and Ven
Diagrams
1. All blocks of cheese are more intelligent than
any philosophy student.
2. Meg the cat is a block of cheese.
3. Therefore, Meg the cat is more intelligent than
any philosophy student.
Blocks of cheese
Things that are more
intelligent than any
philosophy student
Meg the cat
Sound Arguments
•An argument is sound if it is
valid and all of its premises
are true.
•A deductive argument is
unsound if:
– the argument is invalid; or
– one of the premises is untrue.
Truth-Functional Logic
• Decomposes statements into simple
clauses and connecting words
• Example:
Roses are red and violets are blue.
can be broken into 2 simple primitives:
1) Roses are red. 2) Violets are blue.
Along with the connecting word “and”
Fallacious Reasoning
Some Fallacies to Know
Complex Sentences: Conjunctions (The “and”
Connective) & Disjunctions (The “or” Connective)
Complex sentences can be broken down
into simpler sentence structures
some other words & phrases that are
equivalent to “and”
also / both / in addition (to) / including / plus /
along with / all / with / too / as well as / but /
together with
the connective “or” is inclusive; it means:
– one or the other or both
some other words or phrases that are
equivalent to “or” inclusive or exclusive:
Circular Reasoning
• The conclusion is a restatement of the
premises which are meant to support it.
• Example:
– Democracy is the most effective form of
determining how effective something is
because a majority of people voted that
democracy is the most effective form of
determining how effective something is.
Begging the Question
• Definition:
– Endorsing without proof some form of the very
question at issue.
• Example:
Morag: “Why is abortion wrong?”
Hamish: “Because a baby should not be killed simply
because the mother does not want it.”
• Problem:
– The response uses as a justification for the argument
that abortion is wrong the idea that an aborted foetus
is a baby; yet this is one of the fundamental points of
contention in the abortion debate.
– The justifying premises assume the truth of elements of
the conclusion and then use that assumed truth to
justify the premises themselves.
Slippery Slope
• This fallacy consists of objecting to a
particular action on the grounds that
once that action is taken, it will lead
inevitably to a similar but less
desirable action, which will lead in
turn to an even less desirable action,
and so on down the “slippery slope”
until the horror lurking at the bottom
is reached.
• It has been referred to as “the
camel’s nose in the tent”, since
once the camel’s nose enters, the
rest of the camel, it is alleged, will
follow close behind.
Slippery Slope: Example
“If we allow Iran to gain nuclear technology to
build their own power plants, then they will use
the technology to build nuclear bombs.
Before we know it, terrorists all over the world
will have easy access to nuclear bombs.
That’s why Iran must not be allowed to
develop nuclear capabilities!”
• Problems:
– The argument assumes that Iran would not
be satisfied with having nuclear energy and
that they would use the knowledge to build
bombs.
– It also assumes that Iran would then supply
nuclear bombs to any and all terrorists.