Logic and
Critical Thinking
Philosophy 4
Gale Justin
The objective of today’s class is to learn how:
uTo
review the topics that we have so far studied.
uTo
distinguish deductive from inductive arguments.
We will consider each topic in turn.
uWe
begin with the review.
uWe
have already considered the following topics:
Topic 1: Nature of an argument:
An argument is a group of statements, one or more of
which are offered as reason(s) to think that one of the statements is
true.
The statement(s) that are regarded as being the reasons
are called premisses.
The one statement that is being affirmed on the basis of the reason(s)
is called the conclusion.
Topic 2: Arguments vs. Explanations
An argument offers reasons to believe that some statement
is true.
An explanation offers reasons why something happens, has
happened or is the way that it is.
Examples of the distinction between arguments and
explanations:
Argument: Tomatoes will be expensive this year, since
growers in
Explanation: The tomato crop was ruined because heavy
rains rotted the vines of the plants.
To distinguish arguments from explanations, you need to:
uLook
first for the conclusion or the main point of the passage.
uIf
the conclusion or the main point of the passage can be directly known, then the
passage gives an explanation. If the
conclusion or main point can only be indirectly know, ie.
Is an inference, then the passage gives an argument.
uSometimes
there are argument indicators (p. 12).
But sometimes there are not.
Indirectness indicators:
uIndirectness indicators: should, ought, is a good idea, must, may, probably, likely,
apparently, and seems.
uThese words indicate that
the main point in the passage is not directly known. Rather, it is known “through” or on the basis
of the reasons.
uIf no indirectness
indicator is present, ask yourself whether you are being persuaded of the truth
of one statement on the basis of some other statements that are either directly
known or are being taken for granted.
Consider again:
uThe
deftly skinned animal carcasses that have been turning up in the
u(1)
has an indirectness indicator. (2) is
directly known. (3) is being taken for granted.
Topic 3: The conventions that we adopt in order to
structure an argument.
1. Number the statements of the passage in order of their occurrence.
2. Place the number of the conclusion of the argument below the number(s) of
the premisses of the argument.
3. Use an arrow as a conclusion indicator.
Recall that there are two kinds of support:
uIn
some arguments, the reasons/premisses independently
support the conclusion.
uIn
some arguments, the reasons/premisses jointly support
the conclusion.
uIn
some arguments, some of the premisses independently
support the conclusion and some jointly support it.
Case 1: Premisses individually
support the conclusion.
(1) Steve’s car was evidently burgled. (2) Valuables are missing and (3) there are
signs of tampering with the door lock.
Case 2: Premisses support
conclusion only when they are joined.
(1) Busby cannot have set the fire. (2) It broke out at 3
PM. (2) Four of his co-workers saw him at work from 9 am until 5 pm. (4) No one can be in two places at once.
Case 3: Some premisses support
individually, others support only when joined together.
(1) There is a storm system south of Elko. (2) Such systems normally get here the next
day. (3)There are also cloud formations characteristic of an approaching
storm. (4) So it will rain here
tomorrow.
Topic 4: Extracting an argument from its context.
With respect to this topic, you remember that passages
that contain arguments can also contain additional material that neither serves
as the conclusion or as a premiss of the
argument. Such extraneous material
includes background information, repetition, dispensable detail, frills.
Group work on Explanation vs. Arguments and Extracting an argument from its context.
Topic 6: Distinguishing Deductive from Inductive
Arguments.
As the book says, arguments divide into two major
classes, depending on the way in which their conclusions are supported by their
premisses. (p. 24)
Deductive Arguments:
These are arguments in which the premisses
provide conclusive support for the conclusion.
“Conclusive” means that “if the premisses of
the argument are true, then the conclusion of the argument must be true.
Examples of deductive arguments:
All mammals have lungs.
All dogs are mammals.
______________________
Therefore, all dogs have lungs.
The argument has the form:
All A’s are B.
All C’s are A’s.
Therefore, all C’s are B’s.
Inductive Arguments:
The premisses of an inductive
argument establish the conclusion of the argument with some degree of
probability. That is, the premisses provide some,
but not conclusive, support for the conclusion.
Moreover, inductive arguments can vary in the degree of support provided
by the premisses for the conclusion.
Example of an Inductive Argument:
Tomatoes will be expensive this year, since growers in
The premiss provides some
reason to believe the conclusion. But it
does not provide conclusive evidence for the conclusion. The premiss could be true and the conclusion might turn out to
be false.
How to distinguish
Deductive from Inductive Arguments:
The best way is to ask yourself
whether additional information will strengthen or weaken the evidence for the
conclusion. For example, the fact that
we generally import tomatoes from
Some more logical terminology:
uValid: An argument is valid when its form is such
that if all of its premisses are true, then its
conclusion must be true.
uSound:
An argument is sound when its form is valid and all of its premisses
are true.
uNote: “being true or false” is a property of
statements. “being
valid” or “being invalid” is a property of arguments.
Exercise on Distinguishing Inductive from Deductive
Arguments.