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Evaluating arguments - 1
Logic ~ evaluating arguments
What we are doing in this section evaluating arguments
Cf. analyzing argumentsEvaluation - general considerations
1. Are two types of evaluation which applies to all arguments
Evaluating arguments - 2
Logic ~ evaluating arguments
(1) Whether the statements in an argument are true or false. This is not the task of logic
(2) Whether the relationship between the premises & conclusion of an argument is correct. This is the task of logic.
Evaluating arguments - 3
Logic ~ evaluating arguments
Example• Amoebas are not plants, because
they are capable of motion, and no plants have that capacity. – (this is a valid categorical
syllogism)
Evaluating arguments - 4
Logic ~ evaluating arguments
General evaluative considerations (cont’d) 2. The difference in evaluating deductive &
inductive argumentsvalidity refers to the correctness of
the relationship between Ps & C in a deductive argument; is no middle ground
strength refers to the correctness of the relationship between Ps & C in an inductive argument; is middle ground
Evaluating arguments - 5
Logic ~ evaluating arguments
Example of the notion of strength & weakness
• In England, which imports most of its sucrose [sugar], records of the last 100 years show a steady increase in the per capita consumption of sucrose, from about 20 pounds per year in 1820 to over 100 pounds per year today.
Evaluating arguments - 6
Logic ~ evaluating arguments
Example of the notion of strength & weakness (cont’d)
• Present consumption of sucrose in the United States is about the same. This represents 15 to 20 per cent of an individual’s caloric requirements. Concomitant with this increased consumption of sucrose has been an almost parallel rise in the prevalence of caries.
Evaluating arguments - 7
Logic ~ evaluating arguments
Example of the notion of strength & weakness (cont’d)
• Conversely, surveys in Europe and Japan demonstrated that caries were dramatically reduced during periods of wartime restrictions of sugar, syrup, and all sugar products.” (E. Newton, “Sugar and Dental Caries; A Review of Human Studies,” Science 217 (1982): 418.
Evaluating arguments - 8
Logic ~validity
In this section, we shall focus on validity
The notion of validityReview up to this point1. Validity & invalidity apply to
deductive arguments only2. Validity & invalidity is either/or;
there is no middle ground
Evaluating arguments - 9
Logic ~validity
A third trait of validity3. Validity & invalidity are a function
of the form of the argument vs content
Evaluating arguments - 10
Logic ~validity
Example (1) No fideists are rationalists.
Some theists are fideists. Therefore, some theists are not
rationalists.
Evaluating arguments - 11
Logic ~validity
The form of this argument (2) No F are R.
Some T are F. Some T are not R.
Evaluating arguments - 12
Logic ~validity
Can substitute for F, R, & T. (3) No convicts may vote.
Some lawyers are convicts. Therefore, some lawyers are
not eligible to vote.
Since (1) has the same form as (3), if (1) is valid, then (3) is valid.
Evaluating arguments - 13
Logic ~validity, strength & truth
The relationship between validity, strength, & truth
for deductionpremises validity conclusiontrue valid necessarily truetrue invalid may be true or falsefalse valid may be true or falsefalse invalid may be true or false
Evaluating arguments - 14
Logic ~validity , strength & truth
for inductionpremises validity conclusiontrue valid may be true or falsetrue invalid may be true or falsefalse valid may be true or falsefalse invalid may be true or false
Evaluating arguments - 15
Logic ~ on truth
On truth Preliminary clarification
Truth is a property of statementsValidity & invalidity, strength &
weakness are properties of arguments
Evaluating arguments - 16
Logic ~ on truth
Three theories of truth 1. Correspondence
• Some classic defenders: Aristotle (384-322 B.C.), Thomas Aquinas (1225-1274), John Locke (1632-1704)
• A statement is true if it agrees with reality.
Evaluating arguments - 17
Logic ~ on truth
• Strength – Works for many everyday
experiences• Weaknesses
– Does not work well for statements further removed from everyday experience
Evaluating arguments - 18
Logic ~ on truth
– Determining the meaning of correspondence in this context
2. Coherence theory• A classic defender: Francis
Bradley ( 1846-1924); Brand Blanshard (1892-1987)
• A statement is true if it belongs to a set of coherent statements.
Evaluating arguments - 19
Logic ~ on truth
• A look at Bradley’s version of the coherence theory (see reading ~ handout)– He begins with a critique of
foundationalism There are no independent facts– A sound theory of truth does not
need a foundation– Bradley’s alternative: system
Evaluating arguments - 20
Logic ~ on truth
– Two standards for testing the truth of a system
(1) comprehensiveness (2) coherence
Evaluating arguments - 21
Logic ~ on truth
Evaluation of the coherence theory (in general)
• Strength– It can handle statements
distant from everyday experience quite well
Evaluating arguments - 22
Logic ~ on truth
• Weaknesses– How is the truth of systems
established? By coherence with other
systems? The danger of infinite regress?
Evaluating arguments - 23
Logic ~ on truth
– What if a statement which is true in one system conflicts with a statement which is true in another?
3. The pragmatic theory• Some classic defenders: Charles
Sanders Peirce (1838-1914), William James (1842-1910), & John Dewey (1859-1952)
Evaluating arguments - 24
Logic ~ on truth
• A statement is true if it is useful.• Meanings of useful
– makes predictions which are confirmed
– enables us to function in everyday life
– encourages further inquiry– leads to richer & more fulfilling
lives
Evaluating arguments - 25
Logic ~ on truth
• Weaknesses– Broadness of the use of
“useful” Truth is relative -- to individuals and communities
– There are false statements which are useful. E.G. “Wolves are vicious.”
– There seem to be useless truths.
Evaluating arguments - 26
Logic ~ on truth
• Strength – Emphasizes that “truth
matters.”
Evaluating arguments - 27
Logic ~ deductive argument forms
Some common deductive argument forms Argument forms most often used in
everyday argumentation Learning these forms will help us
determine the validity of arguments
Evaluating arguments - 28
Logic ~ deductive argument forms
1. Modus ponens
A B E.G. If the car runs, it
A has gas. B The car runs.
It has gas.
Evaluating arguments - 29
Logic ~ deductive argument forms
1. Modus ponens
A B E.G. If the car runs, it
A has gas. B The car runs.
– It has gas.
Evaluating arguments - 30
Logic ~ deductive argument forms
An invalid form of modus ponens• the fallacy of affirming the consequent
A B E.G. If the car runs, itB has gas. A The car has gas.
– It runs.