Informal Logic Lesson Sample

Below is an outline of a lesson on informal logic delivered in discussion format to several groups of approximately 30 students at the University of North Florida. Chalk board was used for writing examples and logical forms for students to view.

SAMPLE INFORMAL LOGIC LESSON FROM INTRODUCTION TO PHILOSOPHY
By Jennifer Lawson

Claims

When you are confronted with an argument, the first thing you want to do is figure out what kind of claims are being made. This way, you will know how to assess the claims.

A claim can be:

Empirical: The truth or falsity can be assessed by looking into the world.

Example: “All crows are black.”

It would be hard to examine every crow in the world, but if you really want to assess this claim to see if it’s true, you’d look into the world.

A Priori: The truth or falsity cannot be assessed by looking into the world. Usually these claims are said to be assessed by mere reason: thinking about it, examining the concepts.

Example: “A bachelor is an unmarried male.”

Normative: Normative claims are prescriptive. They purport to say what ought to be.

Example: “You should never kill another human.”

Here are some further examples:

My arm is bleeding. (Empirical)
A triangle is a three sided figure. (A priori)
Shouldn’t you make better use of your time? (Normative)
There cannot be a square circle. (A priori)
Smoking increases your risk for heart disease. (Empirical)
Thou shall not covet thy neighbor’s wife. (Normative)

Logic

We make a distinction between inductive and deductive arguments.

An inductive argument is one which, if the premises are true, the conclusion is only probable.

Example:

Every time I have gone to my mom’s house and knocked on the door, her dog, Flip, ran to the door and barked. I am about to knock on my mom’s door. Therefore, Flip will bark.

This is only probable because (A) Flip could have died. (B) Flip could be in the back yard. (C) Flip may have been trained to not bark when someone knocks at the door. Or any number of other reasons. Still, it is probable that Flip will bark next time I knock.

A deductive argument is one which, if the premises are true, the conclusion must be true.

Example:

Socrates is a man
All men are mortal
Therefore, Socrates is mortal

If the premises are true, then the conclusion that Socrates is mortal is necessarily true.

Deductive arguments can be called valid and sound.

When an argument is vaild, that means it’s in a good form. Validity describes a good logical form.

When an argument is sound, that means its form is good and it has true premises.

Here are some examples of vaild argument forms:

Modus Ponens

p→q
p
therefore, q

Modus Tollens

p→q
not q
therefore, not p

Hypothetical Syllogism

p→q
q→r
therefore, p→r

Disjunct

p or q
p
therefore, not q

If the statements put inside these variables are true, the conclusion must be true.

For example, let’s look at Modus Ponens:

If one is a dog, one goes to heaven.
Spot is a dog.
Therefore, Spot goes to heaven.

This is an example of Modus Ponens. This argument is valid. And, if the premises were true, it would also be sound.