Review: Logic

Fallacy: Appeal to Novelty

New is better

The Fallacy of Begging the Question

• The belief in Santa Clause is universal. After all, everyone believes in Santa.

Fallacy of Ignoring the Question

• Mr. Chan: Your homework for tonight is to read pages 1 – 2.

• Student: Why don’t we have more homework?

• Mr. Chan: Yes you can go to the bathroom.

The Fallacy of False Cause

• Every time I don’t shave, we win the game.

The Fallacy of Part and Whole

• All Chans are fit

The Fallacy of Misplaced Authority:

• Student: Mr. Chan my car is dead what should I do?

• Mr. Chan: Well, being a TOK teacher I know everything about cars.

The Fallacy of Accident

• Cutting people with a knife is a crime.

• Surgeons cut people with knives.

• Surgeons are criminals.

The Fallacy of Ad Hominem

• Student: We all know, Mr. Chan, that your argument makes no sense. No self respecting scholar wears a polka dot tie.

The Fallacy of the Double Standard

The Fallacy of Equivocation

A balloon is light.

What is light cannot be dark.

Therefore, a balloon cannot be dark.

The Fallacy of Appeal to Ignorance

• Just because you have not seen Mr. Chan wearing a cape, does not mean he is not superman.

Appeal to the People

My fellow students, as you all know, uniforms hurt learning. We should abolish uniforms.

The Fallacy of False Analogy

• Life is like a box of chocolates

• Therefore it is sweet

Appeal to Ridicule

• Aquinas, how can I respect you as a theologian when you have such an ugly haircut?

Hypothetical Syllogisms• affirming the antecedent

– If A then B– A – Therefore B

• affirming the consequent– If A then B– B– Therefore A

• denying the antecedent– If A then B– Not A– Therefore not B

• denying the consequent– If A then B– Not B– Therefore not A

For example, consider whether this conclusion follows

from the given premises:

If Johnnie eats cake every day, then he is placing himself at risk for diabetes. Johnnie eats cake every day.

Therefore, Johnnie is placing himself at risk for diabetes.

If you think it is valid, you are correct

If Johnnie eats cake every day, then he is placing himself at risk for diabetes.

Johnnie does not eat cake every day.

Therefore, Johnnie is not placing himself at risk for diabetes.

Consider the following

Valid or invalid?

Invalid: He might drink pop every day.

If Johnnie eats cake every day, then he is placing himself at risk for diabetes.

Johnnie is placing himself at risk for diabetes.

Therefore, Johnnie is eating cake every day.

Or, the following:

Invalid: He might be drinking pop every day, or eating chocolate bars, etc.

Or,

If Johnnie eats cake every day, then he is placing himself at risk for diabetes.

Johnnie is not placing himself at risk for diabetes.

Therefore, Johnnie is not eating cake every day.

Valid?

It is valid

Categorical Syllogisms• Grouping into categories.

• Universal Affirmative statements (A statements): the subject is distributed, the predicate is undistributed.

• Universal Negative statements (E statements): both the subject and the predicate are distributed.

• Particular Affirmative statements (I statements): neither subject nor predicate is distributed (both are undistributed).

• Particular Negative statements (O statements): the predicate alone is distributed.

A = All S is P

I = Some S is P

Note the following (bold and underline = distributed):

E = No S is P

O = Some S is not P

Distribution

Categorical Rules:

• In a valid categorical syllogism, the middle term must be distributed at least once.

• In a valid categorical syllogism, any term which is distributed in the conclusion must also be distributed in the premises.

• A syllogism must have three and only three terms. • From two negative premises, no conclusion can be

drawn. • If a premise is particular, the conclusion must be

particular. • If a premise is negative, the conclusion must be

negative.