DEDUCTION & INDUCTION

Formal Logical Argumentation

Deduction

Definition of Deductive Reasoning:

In deductive reasoning, the truth of the conclusion is inherent in the premises.

Deductive Syllogism

The vehicle of deduction is the syllogism:

All men are mortal (Major premise)Socrates is a man (Minor premise)

Socrates is mortal (conclusion )

To test a Syllogism:

To test the validity of a syllogism, you must apply three criteria.

TEST #11.Premises must be truths or possibilities. That is to say, your audience must agree with your major premise and minor premise in order to accept your conclusion.

TEST #1

Gentlemen prefer blondes. Sean Connery is a gentleman. Therefore, Sean Connery prefers blondes.

Does either the major or minor premise seem untrue to you?

Testing a syllogism cont…

TEST #2

2. Must have correct form: AB, CA, CB

All men (A) are mortal (B). Socrates (C) is a man (A).Socrates (C) is mortal (B).

TEST #2

See if you can determine the form of this syllogism. Is it valid?

Example: All murderers have ears. All Methodists have ears. Therefore, all murderers are Methodists.

Answer: All murderers (A) have ears (B). All Methodists (C) have ears (B). Therefore, all murderers (A) are Methodists (C).

Testing a syllogism cont…

TEST #33. Unambiguous language is a necessity. In

other words, all terms must be clearly defined and agreed upon.

TEST #3

Is any of this language ambiguous to you?

Example: Killing an innocent human being is murder. Abortion kills an innocent human being. Therefore, abortion is murder.

Validity is Absolute

Failing any of these three tests renders the syllogism INVALID in its entirety.

Syllogism = Enthymeme

While the syllogism is the form that logic takes in the study of formal logic, this same information is called an enthymeme in rhetoric. The enthymeme is essentially a brief version of the syllogism, one that suits an essay far more effectively.

Example: Since Socrates is a man, he is mortal.

Induction

Inductive Reasoning: Using information about a specific observation (or, empirical data) to reach a general conclusion.

Example: You know, based on a few specific examples, that polar bears are white. Thus, you make an “inductive leap” to conclude that all polar bears are white.

Example: At a traffic light, how do you know it’s safe to accelerate? Because every time you’ve been in that situation, the green light has meant that it’s safe to go.

Testing induction

There are, again, three tests to apply to determine the cogency of any inductive reasoning. Unlike deductive tests, though, inductive reasoning is not absolute. Cogency is determined on a sliding scale.

TEST #1

Known sample: Do you know or can the data be known? Can the data be tested or experimented upon?

TEST #1

Examples from tabloid articles:-87 year old woman pregnant

-Green M&Ms are aphrodisiacs-Aliens built Stonehenge

- Use Crest! Because 3/5 dentists approve

The cogency is determined first by asking whether the data for these conclusions can be known. In these cases, the data is always mysteriously omitted.

Testing inductions cont…

TEST #2

2. Sufficient sample: Is there enough data to make an “inductive leap”?

Test #2

Look at the following conclusions + data. Apply test #2: is this data a sufficient sample to infer the conclusion?

Example: All the corporate industries are crooked (conclusion). Look at Enron (data).

Example: All balding men are wise (conclusion). Look at Dr. Phil (data).

Testing inductions cont…

TEST #3

3. Representative sample: Is it typical of the whole class or things studied?

TEST #3

Example: Conclusion: Everyone thinks it’s a good idea to

put a strip club/bar on campus.

Data: We asked 50 people who all replied, “yes”!

Who is ‘everyone’ in this case? What gender were the people you asked? What age were the people polled? Was every faction (faculty, staff, students) polled in this data collection?

Induction and Deduction

The two modes of reasoning actually quite related. Deductions begin with inductions. That is, the major premise of a syllogism is often a product of inductive reasoning (see example in writing exercise).

“Most of what you know, you know by induction.”

QUESTIONS