Propositional Logic

6.6 Argument Forms and Fallacies

Arguments Forms and Fallacies

Disjunctive Syllogism

p v q / q // ~p … test it with a truth table

p v qq__~p

Not to be confused with…p v q

~p__q

Arguments Forms and Fallacies

Pure Hypothetical Syllogism

Pure Hypothetical Syllogism requires a chain be made where the argument in red fails to make one.

p qr qp r

Not to be confused with…p q

q rp r

Arguments Forms and Fallacies

Modus Ponens (Method of Affirming)

The fallacy in red is called Affirming the Consequent.

p qq___

p

Not to be confused with…p q

p___ q

Arguments Forms and Fallacies

Modus Tollens (Method of Destroying)

The fallacy in red is called Denying the Antecedent.

p q~p__

~q

Not to be confused with…p q

~q___ ~p

Arguments Forms and Fallacies

Constructive Dilemma

Destructive Dilemma

(p q) • (r s) p v r________ q v s

(p q) • (r s) ~q v ~s______ ~p v ~r

p qp___ q

p q~q___ ~p

See Modus Ponens in CD?

See Modus Tollens in DD?

Arguments Forms and Fallacies

If we get Hillary, then we get socialism. If we get Obama, then we get naïveté. We’re going to get either Hillary or Obama as the next

president. So, we’ll either have socialism or naïveté .

Symbolizing…

(H S) • (O N) H v O_________ S v N

Arguments Forms and Fallacies

To refute constructive and destructive dilemmas (you refute both the same way), you either Grasp by the horns Escape between the horns

Grasping by the HornsProve the conjunctive premise false by proving either conjunct false

(H S) • (O N) T F F

Escaping between the HornsProve the disjunctive premise false

For example,

For example,

H v O F F F

Show Hillary is a strong defender of free markets, say

Show, say, Edwards is going to win

Refuting Constructive and Destructive Dilemmas

If we get Giuliani, then we get a philanderer.If we get Huckabee, then we get a religious nut.Either the US doesn’t want a philanderer or it doesn’t

want a religious nut. So, we won’t get Giuliani or we won’t get Huckabee as president.

Symbolizing…

(G P) • (H N)~P v ~N__________~G v ~H

Arguments Forms and Fallacies

To refute constructive and destructive dilemmas (you refute both the same way), you either Grasp by the horns Escape between the horns

Grasping by the HornsProve the conjunctive premise false by proving either conjunct false

(G P) • (H N) F T F

Escaping between the HornsProve the disjunctive premise false

For example,

For example,

~P v ~N F F F

Show Huckabee has reasonable religious views

Show America minds neither philanderers nor religious nuts