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.

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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)

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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

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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.

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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.

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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.

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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

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Logic ~validity

A third trait of validity3. Validity & invalidity are a function

of the form of the argument vs content

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Logic ~validity

Example (1) No fideists are rationalists.

Some theists are fideists. Therefore, some theists are not

rationalists.

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Logic ~validity

The form of this argument (2) No F are R.

Some T are F. Some T are not R.

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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.

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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

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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

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Logic ~ on truth

On truth Preliminary clarification

Truth is a property of statementsValidity & invalidity, strength &

weakness are properties of arguments

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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.

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Logic ~ on truth

• Strength – Works for many everyday

experiences• Weaknesses

– Does not work well for statements further removed from everyday experience

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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.

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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

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Logic ~ on truth

– Two standards for testing the truth of a system

(1) comprehensiveness (2) coherence

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Logic ~ on truth

Evaluation of the coherence theory (in general)

• Strength– It can handle statements

distant from everyday experience quite well

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Logic ~ on truth

• Weaknesses– How is the truth of systems

established? By coherence with other

systems? The danger of infinite regress?

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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)

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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

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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.

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Logic ~ on truth

• Strength – Emphasizes that “truth

matters.”

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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

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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.

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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.

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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.