PHI 1 CRT

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PHI 1 CRT CRITICAL THINKING Norva Y. S.
Lo Philosophy Program, La Trobe University
LECTURE 4 (20/03/2007) Validity
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Two Senses of Reason
A is a justificatory reason (or a justification)
for B iff A justifies B (meaning that if you
accept A then you should also accept B).
Example The reason why you should break off
your engagement with George is that
he is a con artist trying to defraud you.
Statement A You should break off your
engagement with George.
Statement B George is a con artist trying to
defraud you.
It is clear from the passage that statement B is
meant to justify statement A.
We say
Statement B is (put forward as) a justificatory
reason for statement A.
is
Some philosophers prefer to put the matter
slightly differently. They prefer to say
It is the truth of (or the fact stated by)
statement B that justifies the truth of (or the
fact stated by) statement A.
They say
Statement B states a justificatory reason for
that which is stated by statement A.
states
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Example John told the police where his brother
was hiding. I think he did the
right thing. For his brother was a murder
suspect.
Statement A Johns brother was a murder
suspect.
Statement B It was right for John to tell the
police where his brother was hiding.
Interpretation Statement A is meant to be a
justificatory reason for statement B.
Alternatively
A The fact that Johns brother is a murder
suspect.
B Johns action of telling the police where his
brother was hiding.
Interpretation A (the fact that Johns brother
is a murder suspect) is a justi-
ficatory reason for B (Johns action of telling
the police where his brother was
hiding).
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A is a causal-explanatory reason for B iff A
causally explains B (meaning that A is one of the
factors involved in bring about B).
Note
A causal-explanatory reason might not be a
justificatory reason.
Example The reason why Michael bullies other
kids at school is that he has been badly bullied
at home by his older brother. It is very common
for victims of abuse to turn into abusers
themselves. But we all know that what Michael
does is wrong. The fact that one has been abused
does not mean that one has the right to do the
same to others.
Fact A Michael bullies other kids at school.
Fact B Michael has been badly bullied at home
by his older brother.
B is meant to be a reason for A.
What kind of reason ?
It is clear from the passage that B is put
forward as a causal-explanatory reason only, not
a justificatory reason.
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What kind of reason?
The reason why you should quit smoking is that it
is bad for your health. But you know that
already. You just wont quit.
justificatory
causal explanatory
The reason why some people wont believe that
smoking causes cancer is that they want to
continue to enjoy smoking. That is really
irrational.
We can see that the Great Wall is falling down.
Why is it falling down? Because people keep
taking stones from it to build their own houses.
causal explanatory
You should have visited your grand parents at the
weekend because you had promised them that you
would.
justificatory
Mary shouted at Andrew because she was angry at
him. But she really shouldnt have done that.
causal explanatory
Some people dont have children because they
dont like children. Indeed, they shouldnt have
children if they dont like children.
BOTH
The presence of a justificatory reason indicates
the presence of an argument.
Hint
argument
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Summary
A is a justificatory reason for B iff A justifies
B (meaning that if you accept A then you should
also accept B).
A is a causal-explanatory reason for B iff A
causally explains B (meaning that A is one of the
factors involved in bring about B).
Argument df A set of statements, some of
which (the premises) are put forward as
.... reasons to support the others.
justificatory

• A passage must contain some justificatory
  reason in order for it to
• be taken as containing an argument.

• The presence of a causal-explanatory reason
  alone is not a
• sufficient indicator of the presence of an
  argument.

• But as we have seen, in some cases, a
  causal-explanatory reason
• is also put forward as a justificatory
  reason.

• In such cases, an argument is indeed present.

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df That which is allowed (i.e., not ruled
out).
Possibility
Impossibility df That which is ruled out
(i.e., not allowed).
Something is logically possible (or logically
impossible) iff it is allowed (or ruled out) by
the laws of Logic.
Example
A fundamental logical law (Law of
Non-contradiction) says that it is never the case
that P and not-P.

• Is (the existence of) an Italian speaking cat
  logically possible?

• There is nothing self-contradictory about the
  idea of an Italian speaking cat.

allows

• The logical law allows the existence of an
  Italian speaking cat.

We say

• It is logically possible that an Italian
  speaking cat exists.

• It is a logical possibility that an Italian
  speaking cat exists.

• There is a logically possible world which
  contains an Italian speaking cat.

• An Italian speaking cat is a logically possible
  being.

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Example
Is it logically possible for a cat to
simultaneously speak and not speak Italian?
P is logically necessarily true/false
df P is true/false in ALL logically possible
worlds.

• No, because that is a logical contradiction,
  which is ruled out by logic.

Remarks

• logically possible world a world that obeys
  the laws of logic

? a world that you can conceive without
violating logic.

• The actual world obeys logic.

• The actual world is a logically possible world.

• P is logically possibly true/false P is
  true/false in some logically possible world

1. Snow is green.
YES
F
YES
T
YES
YES
2. Snow is white.
F
YES
3. X is married to Y but Y is not married to X.
NO
F
YES
YES
4. Melbourne is the capital of Japan.
5. Cats are not animals.
F
YES
NO
6. Newtons First Law is true.
YES
YES
T
7. Everything is identical to itself.
T
NO
YES
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Something is logically possible (or logically
impossible) iff it is allowed (or ruled out) by
the laws of Logic.
Something is physically possible (or physically
impossible) iff it is allowed (or ruled out) by
the laws of the physical sciences.
Something is legally possible (or legally
impossible) in Australia iff it is allowed (or
ruled out) by the laws of the Australian legal
system.
Something is economically possible principles
of economics.
That we can stop a moving object (its mass being
constant) without applying any force to it.
Something is psychologically possible
principles of psychology.
Gay marriage
Something is politically possible political
interests, constrains.
Something is . possible
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Note It is commonly thought - All physical
sciences implicitly presuppose (i.e., must not
violate) laws of logic. - All legal or political
systems implicitly presuppose (i.e., must not
violate) laws of the physical sciences.
That is why physical possibilities are perceived
as a subset of logical possibilities.
That is why both legal possibilities and
political possibilities are perceived as subsets
of logical possibilities.
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Valid Argument
(valid inference)
inference
An argument from P to C is valid
P logically guarantees C.
df
It is not logically possible that P is true but
at the same time C is false.
P is true but at the same time C is
false.
The combination of P being true and C being false
is logically impossible.
P1. IF John is not at work THEN he is ill.
P. John is a married man.
__________________________
P2. John is not at work.
valid
___________________________________________
valid
C. John is a man.
C. John is ill.
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P. John wears a wedding ring.
_______________________________
invalid
C. John is a married man.
P1. IF It is raining THEN the ground is wet.
P2. The ground is wet.
___________________________________________
invalid
C. It is raining.
Readings for this week Chapter 2 and Chapter 10,
Elements of Reasoning.