# Propositional Logic

Rosen 6 th ed., § 1.1-1.2

1

### Foundations of Logic: Overview

• Propositional logic: – Basic definitions. – Equivalence rules & derivations. • Predicate logic – Predicates.

– Quantified predicate expressions.

– Equivalences & derivations.

2

### Propositional Logic

Propositional Logic

is the logic of compound statements built from simpler statements using

Boolean connectives.

Applications: • Design of digital electronic circuits.

• Expressing conditions in programs.

• Queries to databases & search engines.

3

### Proposition

A

proposition

(

p

,

q

,

r

, …) is simply a

statement

(

i.e.

, a declarative sentence)

with a definite meaning

, having a

truth value

that’s either

true

(T) or

false

(F) (

never

both, neither, or somewhere in between).

[In

probability theory,

we assign

degrees of certainty

to propositions. For now: True/False only!] 4

### Examples of Propositions

• “It is raining.” (Given a situation.) • “Beijing is the capital of China.” • “1 + 2 = 3” The following are

NOT

propositions: • “Who’s there?” (interrogative, question) • “La la la la la.” (meaningless interjection) • “Just do it!” (imperative, command) • “Yeah, I sorta dunno, whatever...” (vague) • “1 + 2” (expression with a non-true/false value) 5

### Operators / Connectives

An

operator

or

connective

combines one or more

operand

expressions into a larger expression. (

E.g.

, “+” in numeric exprs.)

Unary

operators take 1 operand (

e.g., binary

operators take 2 operands (

eg

-3); 3  4).

Propositional

or

Boolean

operators operate on propositions or truth values instead of on numbers.

6

### The Negation Operator

The unary

negation operator

“¬” (

NOT

) transforms a prop. into its logical

negation

.

E.g.

If

p

= “I have brown hair.” then ¬

p

= “I do

not

have brown hair.”

Truth table

for NOT:

p

p

T F F T 7

### The Conjunction Operator

The binary

conjunction operator

“  ” (

AND

) combines two propositions to form their logical

conjunction

.

E.g.

If

p

=“I will have salad for lunch.” and

q=

“I will have steak for dinner.”, then

p

q

=“I will have salad for lunch

and

I will have steak for dinner.” 8

### Conjunction Truth Table

• Note that a

p

F

q

F

p

q

F F T T F F F i.e., sufficient to express

any

truth table!

9

### The Disjunction Operator

The binary

disjunction operator

“  ” (

OR

) combines two propositions to form their logical

disjunction

.

p

=“That car has a bad engine.”

q=

“That car has a bad carburetor.”

p

q

=“Either that car has a bad engine,

or

that car has a bad carburetor.” 10

### Disjunction Truth Table

• Note that

p

q

means

p q p

q

that

p

is true, or

q

is true,

or both

are true!

F F F • So this operation is also called

inclusive or,

F T T F

T T

T T T because it

includes

the possibility that both

p

and

q

are true.

• “¬” and “  ” together are also universal.

11

### A Simple Exercise

Let

p

=“It rained last night”,

q

=“The sprinklers came on last night,”

r

=“The lawn was wet this morning.” Translate each of the following into English: ¬

p r

 ¬

p

¬

r

p

= = 

q =

“It didn’t rain last night.” “The lawn was wet this morning, and it didn’t rain last night.” “Either the lawn wasn’t wet this morning, or it rained last night, or the sprinklers came on last night.” 12

### Operator

The binary

exclusive-or operator

“  ” (

XOR

) combines two propositions to form their logical “exclusive or” (exjunction?).

p

= “I will earn an A in this course,”

q p

= “I will drop this course,” 

q

= “I will either earn an A for this course, or I will drop it (but not both!)” 13

### Exclusive-Or Truth Table

• Note that

p

q

means

p q p

q

that

p

is true, or

q

is true, but

not both

!

• This operation is F F F T T F F T T called

exclusive or,

because it

excludes

the T T

F

possibility that both

p

and

q

• “¬” and “  ” together are

not

are true.

universal.

14

### Natural Language is Ambiguous

Note that English “or” is

by itself

ambiguous regarding the “both” case!

“Pat is a singer or Pat is a writer.” 

p q p

F F F T or F T

q

“Pat is a man or Pat is a woman.”  T F T T T undef.

Need context to disambiguate the meaning!

For this class, assume “or” means inclusive.

15

### Operator

The

implication p

q

states that

p

implies

q.

It is FALSE only in the case that p is TRUE but q is FALSE.

E.g.

,

p

=“I am elected.”

q

=“I will lower taxes.”

p

q =

“If I am elected, then I will lower taxes” (else it could go either way) 16

### Implication Truth Table

• • • •

p

q

is

false

only when

p

is true but

q p

q

does is

not not

true.

imply that

p p

 causes

q

!

q

does

not

imply

p q p

q

F F F T T F T T that

p E.g.

or q are ever true !

“(1=0)  pigs can fly” is TRUE!

T T F T 17

### Examples of Implications

• “If this lecture ends, then the sun will rise tomorrow.”

True

or

False

?

• “If Tuesday is a day of the week, then I am a penguin.”

True

or

False

?

• “If 1+1=6, then George passed the exam.”

True

or

False

?

• “If the moon is made of green cheese, then I am richer than Bill Gates.”

True

or

False

?

18

Inverse, Converse, Contrapositive Some terminology: • The

inverse

of

p

 • The

converse q

of

p

 is: ¬

q

is:

p q

  ¬

p

.

q

• The

contrapositive

of

p

q

is: ¬

q

 ¬

p.

• One of these has the

same meaning

truth table) as

p

q

(same . Can you figure out which?

19

### How do we know for sure?

Proving the equivalence of

p

q

and its contrapositive using truth tables:

p q

q

p p

q

q



p

F F F T T F T T T F T F T T F F T T F T T T F T 20

### operator

The

biconditional p

and only if (IFF) q

is true.

It is TRUE when both

p

q

and

q

 TRUE.

q

states that

p

is true

if p

are

p

= “It is raining.”

q

= “The home team wins.”

p

q =

“If and only if it is raining, the home team wins.” 21

### Biconditional Truth Table

p

q

means that

p

and

q

have the

same

truth value.

p q p

• • Note this truth table is the exact

opposite

of  ’s!

p

q

means ¬(

p

q

)

p

q

does

not

imply F F F T T F T T

p

and

q

are true, or cause each other.

 T F F T

q

22

### Boolean Operations Summary

• We have seen 1 unary operator (4 possible) and 5 binary operators (16 possible).

p q

p p

q p

q p

q p

q p

q

F F T F T T T F F T T F F F F T F T T T F T T F T T F T T F F T 23

### Precedence of Logical Operators

Operator ¬     Precedence 1 2 3 4 5 24

### Nested Propositional Expressions

• Use parentheses to

group sub-expressions

: “I just saw my old

f

riend, and either he’s

g

rown or I’ve

s

hrunk.” =

f

 (

g

s

) – (

f

f

 

g g

)  

s s

would mean something different would be ambiguous • By convention, “¬” takes

precedence

both “  ” and “  ”.

– ¬

s

f

means (¬

s

) 

f

,

not

¬ (

s

 over

f

) 25

### Some Alternative Notations

Name: Propositional logic: Boolean algebra: C/C++/Java (wordwise): C/C++/Java (bitwise): not and or xor implies  

p pq

  +   ! && || != ~ & | ^ Logic gates: iff  == 26

A

tautology

is a compound proposition that is true no matter what the truth values of its atomic propositions are!

Ex. p

 

p

[What is its truth table?] A

is a comp. prop. that is

false

no matter what!

Ex. p

 

p

[Truth table?] Other comp. props. are

contingencies

.

30

### Propositional Equivalence

Two

syntactically

(

i.e.,

textually) different compound propositions may be

semantically

identical (

i.e.,

have the same meaning). We call them

equivalent

. Learn: • Various

equivalence rules

or

laws

.

• How to

prove

equivalences using

symbolic derivations

.

31

### Proving Equivalences

Compound propositions

p

and

q

are logically equivalent to each other IFF p and

q

contain the same truth values as each other in all rows of their truth tables.

Compound proposition

p

is

logically equivalent

to compound proposition

q

, written

p

q

,

IFF

the compound proposition

p

q

is a tautology.

32

Proving Equivalence via Truth Tables

Ex.

Prove that

p

q

  ( 

p

 

q

).

p q

F F F T T F T T

p

q

F T T T 

p

T T F F 

q

T F T F 

p

 

q

T F F F  ( ( 

p

 

q

) ) F T T T 33

### Equivalence Laws

• These are similar to the arithmetic identities you may have learned in algebra, but for propositional equivalences instead.

• They provide a pattern or template that can be used to match much more complicated propositions and to find equivalences for them.

34

### Equivalence Laws - Examples

• • • • • •

Identity

:

p

T

Domination

:

p

T

p p

F

T p

F

p

F

Idempotent

:

p

p

p p

p

Double negation:



p

p p Commutative: p

q

q

p p

q

q

p Associative:

(

p

q

) 

r

(

p

q

) 

r

 

p

 (

q

r

)

p

 (

q

r

) 35

Distributive

:

p

 (

q

r

) 

p

 (

q

r

)  (

p

q

)  (

p

r

) (

p

q

)  (

p

r

) •

De Morgan’s

:  (

p

q

)  

p

 

q

 (

p

q

)  

p

 

q

36

### More Equivalence Laws

Absorption

:

p

 (

p

p

 (

p

q

) 

q

) 

p p

:

p

 

p

T

p

 

p

F

37

Defining Operators via Equivalences Using equivalences, we can

define

operators in terms of other operators.

• Implication:

p

q

 

p

 • Biconditional:

p

q

 (

p

q

)

p

q

  (

p

q

) 

q

(

q

p

) • Exclusive or:

p

q p

q

  (

p

q

)  (

p

q

) (

p



q

)  (

q



p

) 38

### An Example Problem

• Check using a symbolic derivation whether (

p

 

q

)  (

p

r

)  

p

q

 

r

.

(

p

 

q

)  [Defn. of  (

p

[Expand definition of ] 

r

)   ( 

p

 ]   

q

( )

p

   ( (

p q

 )  (

p

r

)

r

)   (

p

r

) ) [DeMorgan’s Law]  ( 

p

q

)  ((

p

r

)   (

p

r

)) 39

### Example Continued...

( 

p

  (

q q

)   

p

) ((

p

 

q

 ( 

p

 ((  ((

p r

)   (

p

r

)    (

p r

)) 

p

r

)   (

p

r

 )) [ [   commutes] associative]

r

))) [distrib.  over  ] 

q

 ((( 

p

[assoc.] 

q

  [trivial taut.]  (

p

 ((( 

p r

))  

p

) (  

p r

)    ( ( 

p p

r

)))   (

p q

 ((

T

r

)  ( 

p

  (

p

 

r

)))

r

))) [domination]  [identity] 

q q

  (

T

( 

p

 ( 

p

  (

p

  (

p

r

))  

r

)))

cont.

40

### End of Long Example

q

 ( 

p

  (

p

 [DeMorgan’s] 

r

))

q

 [Assoc.]  [Idempotent] 

q q

  ( 

p

(( 

p

 ( 

p

 

r

))  

p

)  

r

) ( 

p

 

r

) [Assoc.]  (

q

 

p

)  

r

[Commut.]  

p

q

 

r Q.E.D. (quod erat demonstrandum)

41