Critical Thinking Ch 1D: Analyzing Arguments - Inductive and Deductive

Inductive Argument

An inductive argument assumes that what has been true for some/most will be

true for all.

• It makes a case for a general conclusion from more specific premises.

Inductive Argument Examples

Ex 1: Most earthquakes I have felt have occurred at night. The next earthquake to hit LA will

be at night.

Ex 2: Martha is a grandmother, so Martha is old.

Ex 3: I won the last five times I gambled. Thus, I have luck on my side and will continue to win!

Evaluating an Inductive Argument

The argument is strong if it makes a compelling case for its conclusion. (Built on

much valid evidence.)

The argument is weak if its conclusion is not well supported by its premises. (Built

on little or faulty evidence.)

An inductive argument cannot prove its conclusion true. The argument can only be evaluated in terms of its strength.

Deductive Argument

A deductive argument assumes that what is true for all members of the set

must be true for each member if the set.

A deductive argument makes a case for a specific conclusion from more general

premises.

Deductive Argument Examples

Ex 1: Premise: An island is surrounded by water.

Premise: Hawaii is an island.

Conclusion: Hawaii is surrounded by water

Ex 2: An island is surrounded by water. Santa Clarita is not surrounded by water and is thus not an island. (Valid)

Ex 3: An island is surrounded by water. Antarctica is surrounded by water and is thus an island. (Invalid)

An argument is VALID

If the conclusion follows necessarily from its premises regardless of the truth of the premises or conclusion.

If the conclusion does not follow necessarily from the premises, then the argument is

invalid

If the argument is valid, then it is impossible for the premises to be true and the conclusion to

be false

An argument is SOUND

If it is a valid argument and its premises are all true

Deductive Examples

Conditional Deductive Arguments

Affirming the Conclusion Examples (Consequent)

q is true, thus p is true → invalid

The truth of the conclusion doesn’t prove the truth of the hypothesis.

Denying the Hypothesis Examples (Antecedent)

• p is not true, thus q is not true → invalid

• If the premise is not true, it doesn’t prove the conclusion also not true.

Denying the Conclusion Examples (Consequent)

• q is not true, thus p is not true → valid

• If the conclusion is not true, it proves the premise is not true.