This document discusses evaluating arguments. It provides guidelines for determining whether an argument is good and whether its premises are acceptable. A good argument must be deductively valid or inductively strong, clear, precise, relevant, consistent, complete and fair. Premises must be true for the argument to be sound. Background beliefs and testimony from credible sources can provide evidence for accepting premises. Arguments can be refuted by showing a critical premise is false or dubious, or that the conclusion does not follow from the premises.

1. Ch 8
Evaluating Arguments
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2.  Once you have an argument
summarized/standardized, you need to evaluate it
to see if you are forced to accept the conclusion.
 There are two main questions to ask when doing
so:
 Is the argument a “good argument”?
 Are the premises acceptable?
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3.  An argument is NOT good simply because it:
 “agrees with my views”
The attitude that only arguments that agree with
your viewpoints are good is extremely close-minded.
 “is a persuasive argument”
People aren’t always smart and can be persuaded
by “eloquent speech” (and be confused by solid
reasoning).
Hitler was more persuasive than Churchill, but that
doesn’t mean that Hitler’s arguments were better.
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4.  “is well-written/spoken”
 Although it’s easier to tell whether an argument
is good if it is well written, being well written
doesn’t make it good. Clarity, eloquence and
organization can all occur in the presence of
logical mistakes.
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5.  It must, at the least, be either deductively sound (valid
with true premises) or inductively cogent (strong with
true premises).
 It will also need to be clear…
 An argument isn’t good unless it is understandable.
 …precise…
 One needs to avoid equivocation and use exact language.
 …the premises need to be relevant…
 Arguments with a lot of irrelevant material can’t be said to
be good arguments.
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6.  …consistent…
 Arguments that contain logical contradictions commit the
fallacy of inconsistency.
 …complete…
 If an arguer ignores facts relevant to the conclusion at hand,
we can’t say the argument is good (it doesn’t account for
relevant objections).
 …and fair.
 An argument can’t be good if it hastily dismissed objections.
 (See guidelines on p. 206.)
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7.  Arguments always contain premises - some premises will
have support from other premises,—there are others that are
mere assumptions (claims made by the arguer).
 If the argument is valid/ strong, its soundness/cogency will
turn on whether these assumptions are true.
 So how can we tell if we should accept them?
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8.  It is reasonable to accept a claim if…
 (1) The claim does not conflict with personal
experience that we have no good reason to doubt it.
 (2) The claim does not conflict with background beliefs
that we have no good reason to doubt.
 (3) The claim comes from a credible source.
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9.  In general, you should favor the testimony of your own sight
(and other senses) over the testimony of others.
 Someone tells you her Doberman - gentle as a kitten.
 You’ve seen him attack many people.
 You should probably not believe that the Doberman is gentle.
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10.  However, your senses are not indubitable (un-doubtable).
 They can be mistaken for any number of reasons such as:
 Bad physical conditions (e.g., poor lighting)
 Sensory impairment (e.g., poor vision)
 Observer impairment (e.g., drunk)
 Unreliable measuring instruments
 Bad memory
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11. 11

12. 12

13. Just how are
are you supposed to
to count how many times
“are” appears in this quote?
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14.  Background beliefs: convictions held—usually assumed
without question.
 e.g., It snowed in Las Vegas last July 4th
.
This seems to contradict our background belief that
it doesn’t snow in deserts during the summer.
 e.g., George W. Bush is a robot.
This contradicts our background belief that people
aren’t robots.
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15.  Nonetheless background beliefs should be
subject to revision if sufficient evidence is
presented against them (don’t be dogmatic about
any beliefs you have).
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16.  Much of what we believe is based on the
testimony of sources.
 We saw a lot about this in chapter 6 (more is in
chapter 12).
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17.  Questions to ask to determine source credibility:
 Genuine expert?
 Are they outside their area?
 Are they biased?
 Do they have a reason to lie?
 Questionable senses (were they drunk)?
 Are they generally reliable (is it The Enquirer? ) Right
context?
 Can expert opinion settle the issue (e.g., is this a moral
issue)?
 Is it improbable?
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18.  There are two ways to refute an argument:
 Show that a premise—or a critical group of premises—
is false or dubious.
 Show that the conclusion does not follow from the
premises.
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19.  If a premise is critical to an argument, showing it false
will refute the argument.
 (1) All presidents live in the White House. (2) Paris
Hilton is President. So, (3) Paris Hilton lives in the
White House.
 Showing (2) to be false is sufficient to refute the
argument.
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20.  However, showing false an irrelevant premise will not
refute the argument.
 (1) All circles are squares. (2) All squares are
rectangles. (3) All rectangles are geometrical figures.
(4) So, all squares are geometrical figures.
 Showing (1) is false won’t keep (2) and (3) from
proving (4).
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21.  Additionally, for refutation, the premise must be
necessary (critical)
 (1) TJ is a bachelor. (2) TJ is an uncle. (3) So, TJ is a
Male.
 Since both (1) and (2) provide independent support
for (3) falsifying only one of them will not refute the
argument.
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22.  If a critical premise is shown to be doubtable, then the
argument has been refuted (by showing it to be
unconvincing).
 Demonstrating doubt:
 Appeal to personal experience, common knowledge, or
reputable source.
 Note a self-contradiction (either in a single premise or between
premises).
 Show the premises is based on an unwarranted assumption.
 Personally demonstrate its falsity or dubiousness.
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23.  Other refutation Techniques:
 Reducing to the absurd: Show the truth of a premise
would entail something clearly false (absurd).
 Present a counter-example: present an exception that
shows a premise false.
(arguer): All 20th
Century presidents were rich.
(you): Harry Truman wasn’t!
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24.  To do this, you need to show that the argument is either
(a) deductively invalid or (b) inductively weak.
 Most important questions:
 If deductive, does the conclusion follow necessarily
from the premises?
 Are the premises relevant (is there a fallacy)?
 Are the premises sufficient to support the conclusion?
 Does the argument omit any crucial countervailing
evidence?
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25.  Exercise 8.3 – Parts I, II and III
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