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Dilemma: Division Into Cases. Dilemma: p Ú q p ® r q ® r r Premises: x is positive or x is negative. If x is positive , then x 2 is positive. If x is negative, then x 2 is positive. Conclusion: x 2 is positive. Application: Find My Glasses.

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dilemma division into cases
Dilemma: Division Into Cases

Dilemma: p Ú q

p ® r

q ® r

\ r

Premises:x is positive or x is negative.

If x is positive , then x2 is positive.

If x is negative, then x2 is positive.

Conclusion:x2 is positive.

application find my glasses
Application: Find My Glasses

1. If my glasses are on the kitchen table, then I saw them at breakfast.

2. I was reading in the kitchen or I was reading in the living room.

3. If I was reading in the living room, then my glasses are on the coffee table.

4. I did not see my glasses at breakfast.

5. If I was reading in bed, then my glasses are on the bed table.

6. If I was reading in the kitchen, then my glasses are on the kitchen table.

find my glasses cont d
Find My Glasses (cont’d.)

Let: p = My glasses are on the kitchen table.

q = I saw my glasses at breakfast.

r = I was reading in the living room.

s = I was reading in the kitchen.

t = My glasses are on the coffee table.

u = I was reading in bed.

v = My glasses are on the bed table.

find my glasses cont d4
Find My Glasses (cont’d.)

Then the original statements become:

1. p ® q 2. r Ú s 3. r ® t

4. ~q 5. u ® v 6. s ® p

and we can deduce (why?):

1. p ® q 2. s ® p 3. r Ú s 4. r ® t

~q ~p ~s r

\ ~p \ ~s \ r \ t

Hence the glasses are on the coffee table!

fallacies
Fallacies
  • A fallacy is an error in reasoning that results in an invalid argument.
  • Three common fallacies:
    • Using vague or ambiguous premises;
    • Begging the question;
    • Jumping to a conclusion.
  • Two dangerous fallacies:
    • Converse error;
    • Inverse error.
converse error
Converse Error

If Zeke cheats, then he sits in the back row.

Zeke sits in the back row.

\ Zeke cheats.

  • The fallacy here is caused by replacing the impication (Zeke cheats ® sits in back) with its biconditional form (Zeke cheats « sits in back), implying the converse (sits in back ® Zeke cheats).
inverse error
Inverse Error

If Zeke cheats, then he sits in the back row.

Zeke does not cheat.

\ Zeke does not sit in the back row.

  • The fallacy here is caused by replacing the impication (Zeke cheats ® sits in back) with its inverse form (Zeke does not cheat ® does not sit in back), instead of the contrapositive (does not sit in back ® Zeke does not cheat).
universal instantiation
Universal Instantiation
  • Consider the following statement:All men are mortal Socrates is a man. Therefore, Socrates is mortal.
  • This argument form is valid and is called universal instantiation.
  • In summary, it states that if P(x) is true for all xÎD and if aÎD, then P(a) must be true.
universal modus ponens
Universal Modus Ponens
  • Formal Version: "xÎD, if P(x), then Q(x).P(a) for some aÎD. \Q(a).
  • Informal Version: If x makes P(x) true, then x makes Q(x) true.a makes P(x) true.\a makes Q(x) true.
  • The first line is called the major premise and the second line is the minor premise.
universal modus tollens
Universal Modus Tollens
  • Formal Version: "xÎD, if P(x), then Q(x).~Q(a) for some aÎD. \~P(a).
  • Informal Version: If x makes P(x) true, then x makes Q(x) true.a makes Q(x) false.\a makes P(x) false.
examples
Examples
  • Universal Modus Ponens or Tollens???

If a number is even, then its square is even.

10 is even.

Therefore, 100 is even.

If a number is even, then its square is even.

25 is odd.

Therefore, 5 is odd.

using diagrams to show validity
Using Diagrams to Show Validity
  • Does this diagram portray the argument of the second slide?

Mortals

Men

Socrates

modus ponens in pictures
Modus Ponens in Pictures
  • For all x, P(x) implies Q(x).P(a).Therefore, Q(a).

{x | Q(x)}

{x | P(x)}

a

a modus tollens example
A Modus Tollens Example
  • All humans are mortal.Zeus is not mortal.Therefore, Zeus is not human.

Zeus

Mortals

Humans

modus tollens in pictures
Modus Tollens in Pictures
  • For all x, P(x) implies Q(x).~Q(a).Therefore, ~P(a).

{x | Q(x)}

a

{x | P(x)}

converse error in pictures
Converse Error in Pictures
  • All humans are mortal.Felix the cat is mortal.Therefore, Felix the cat is human.

Mortals

Felix?

Humans

Felix?

inverse error in pictures
Inverse Error in Pictures
  • All humans are mortal.Felix the cat is not human.Therefore, Felix the cat is not mortal.

Mortals

Felix?

Felix?

Humans

quantified form of converse and inverse errors
Quantified Form of Converseand Inverse Errors
  • Converse Error:" x, P(x) implies Q(x).Q(a), for a particular a.\P(a).
  • Inverse Error:" x, P(x) implies Q(x).~P(a), for a particular a.\~Q(a).
an argument with no
An Argument with “No”
  • Major Premise: No Naturals are negative.
  • Minor Premise:k is a negative number.
  • Conclusion:k is not a Natural number.

Negative numbers

Natural numbers

k

abduction
Abduction
  • Major Premise: All thieves go to Paul’s Bar.

Minor Premise: Tom goes to Paul’s Bar.

Converse Error: Therefore, Tom is a thief.

  • Although we can’t conclude decisively if Tom is a thief or not, if we have further information that 99 of the 100 people in Paul’s Bar are thieves, then the odds are that Tom is a thief and the converse error is actually valid here.
  • This is called abduction by Artificial Intelligence researchers.
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