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Logic

Laws of Logic

QuestionAnswer
MODUS PONENS (MP) -- direct reasoning applies to conditionals If it rains, we will go to the movies. It rains. THEREFORE, we go to the movies. P-->Q and P, then Q
MODUS TOLLENS (MT) indirect reasoning applies to conditional) If it rains, we will go to the movies. We don't go to the movies. THEREFORE, it didn't rain. P-->Q and ~Q, then ~P. (works because of the logical equivalence of the contrapositive)
LAW OF SYLLOGISM, a.k.a. "transitive property of conditionals" P-->Q and Q-->R, then P-->R
Simplification - applies to AND statements only (CONJUNCTIONS) If you know P˄Q, then you can say P by itself or Q by itself, they're both true!!
Equivalent Disjunctive (ED) -- magically turns a conditional statement into a disjunction OR P --> Q ≡ ~P V Q
Disjunctive Syllogism - applies only to a DISJUNCTION Either we go to the movies, or we go bowling. We don't go to the movies. Then we must have gone bowling! PVQ and ~P, then Q
Law of Contrapositive P --> Q ≡ ~Q --> ~P
The inverse and the converse of a conditional statement are logically equivalent Because they're contrapositives of each other! Inverse: ~P --> ~Q Converse: Q --> P
When is a disjunction false? Only one instance... F V F = F
When is a conjunction true? Only one instance.... T ˄ T = T
When is a conditional false? Only one instance, the broken promise... T --> F = F
To negate a conditional, first rewrite it as a disjunction using th equivalent dusjunctive... Then use Demorgan's Law: ~P--> Q) ≡ ~(~P V Q) ≡ P˄~Q
Other ways to say P --> Q "If P, then Q" 1. P implies Q 2. Q if P 3. P only if Q
When is the biconditional true? Only two instances... When the two statements are the SAME truth value, i.e. T <--> T, F <--> F
To translate the biconditional P <--> Q into words P if and only if Q
DEMORGAN'S LAW - negates conjunctions and disjunctions ~(PVQ)≡ ~P ˄ ~Q ~(P˄Q)≡ ~P V ~Q
The associative property only applies when all the conjunction/disjunction symbols are the same. Reassociate the parentheses (A V B) V C ≡ A V (B V C) (A ˄ B) ˄ C ≡ A ˄ (B ˄ C)
Distributive property for conjuctions/disjunctions A V (B ˄ C)≡ (AVB)˄(AVC) A ˄ (B V C)≡ (A˄B)V(A˄C)
Created by: kgkasper
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