Phil
106: Critical Thinking

LARKIN

Southern
Illinois University Edwardsville

** **

**Deductive
Concepts**

I.
Deduction
vs. Induction

A.
**Deductive** Form: The premises are intended to provide *conclusive* reasons
or proof of the conclusion.

B.
**Inductive** Form: The premises are intended to provide *compelling* but not
conclusive reasons for the conclusion.

II.
Validity

A.
Good
Deductive Form = Validity

B.
Definitions
(these definitions are just two different ways of saying the same thing)

1.
An
argument is ** valid** =df If all the premises are true, then the
conclusion

*2.
*An argument is ** valid** =df It is

* *

C.
Validity
(in the technical sense just defined) applies only to arguments, never to
individual claims.

D.
Validity
is completely determined by an argument’s structure, not its content. If some argument is valid, then every
argument with the same structure is also valid.

* *

III.
Soundness

A.
Good
Deductive Form + Good Content = Soundness

B.
Definition:

An argument is ** sound** =df It is valid
and has all true premises.

C.
If
an argument has one or more false premises or it is not valid, then the
argument is not sound.

D.
Like
validity, soundness (in the technical sense just defined) applies only to
arguments, never to individual statements/claims.

IV.
True/False
Questions

1.
A
valid argument must have a true conclusion

**FALSE**: A valid
argument must have a true conclusion only if all of the premises are true. So it is possible for a valid argument to
have a false conclusion as long as at least one premise is false.

2.
A
sound argument must have a true conclusion.

**TRUE**: If an
argument is sound, then it is valid and has all true premises. Since it is valid, the argument is such that
*if* all the premises are true, then the conclusion must be true. A sound argument really does have all true
premises so it does actually follow that its conclusion must be true.

3.
If
a valid argument has a false conclusion, then at least one premise must be
false.

**TRUE**: A valid
argument cannot have all true premises and a false conclusion. So if a valid argument does have a false
conclusion, it cannot have all true premises.
Thus at least one premise must be false.

4.
If
an invalid argument has all true premises, then the conclusion must be false.

**FALSE**: It is
possible for an invalid argument to have all true premises and a true
conclusion.

Ex: P1: All dogs are
mammals.

P2: All terriers are mammals.

C: All terriers are dogs.

This argument really does have all true premises and
a true conclusion, but still it is invalid—because it is *possible* for an
argument with this structure to have true premises and a false conclusion:

Ex: P1: All dogs are mammals.

P2: All cats are mammals.

C: All cats are dogs.

5.
If
an argument has all true premises and a true conclusion, then it is valid.

**FALSE**: It is
possible for an argument to have all true premises and a true conclusion but
still be invalid. See above (#4).

6.
If
an argument has all true premises and a false conclusion, then it is invalid.

**TRUE**: A valid
argument cannot possibly have all true premises and a false conclusion. If some argument really does have all true
premises and a false conclusion, then it is obviously possible for such an
argument to have true premises and a false conclusion. So the argument is invalid.