LPS 29 Lecture Notes - Lecture 4: Propositional Calculus, Wnew-Fm, Logical Form

32 views4 pages
Page:
of 4
LPS 29 - Lecture 3 Propositional Logic
3.3 Formalization
Language of propositional logic: language consisting of symbolic notation
Formalization turns a word sentence into a sentence or argument form
Formalization of a simple sentence
o Today is cold. (P)
o Today is not cold. (~P)
Formalization of sentences with several logical operators
o Example 1: Today is not both very hot (P) and very cold (Q)
~ (P & Q)
Note: Brackets are needed in this case because ~ acts as a negative
sign, and only applied to the sentence letter it is attached to.
~P & Q means: Today is not very hot and today it is very cold
o Example 2: Today it is either hot (P) and sunny (Q) or cold (R) and windy (S).
(P & Q) V (R & S)
Note: Brackets must specify which sentence letters fit together to properly
reflect the meaning of the sentence
“P & Q V R & S” could mean different things such as
o (P & (Q V (R & S))), ((P & Q V R) &S)
o Both of these have a different meaning than (P & Q) V (R
& S)
Review: Vocabulary of language of propositional logic:
o Sentence Letters: Capital letters that represent a specific sentence/part of a
sentence. Can contain numerical subscripts.
o Logical Operators: ~, &, V, ,
o Brackets: (,)
Sentence letters are nonlogical symbols because they can represent different sentences
Logical Operators and Brackets are logical symbols because they always represent the
same thing
Well-formed Formula (wwf): Formulas that are meaningful
o Ex: (P V (Q & R)) is a wwf
o Ex: ((V P & R) S) is not a wwf
Three Rules that Qualify a WFF
o Any sentence letter is a WFF
o If P is a WFF, then its negation, ~P is a WFF
o If P and Q are WFFs, then so are sentences with logical operators that put them
together. (P & Q), (P V Q), (PQ), (P Q)
WWFs cont…
o Note: a form with many negation signs is a WFF!
~~~~~~~~~~~~~~P is a WFF
Even numbers of ~ means it is not a negation
Odd numbers of ~ means it is a negation
o Note: Always bracket wwfs with binary operators (V & )
WWFS that are not bracketed are called unofficial and not really WWFS
o Atomic WWFs: Sentence letters (ex. P, Q, R, S)
find more resources at oneclass.com
find more resources at oneclass.com
o Molecular/Compound WFFS: all other wffs that aren’t atomic.
o Subwff: part of a wff that is itself a wff [ex. (~P & R) in this case, P is a subwff of
(~P & R)]
o Main operator: the binary operator that includes the scope of the entire formula
Ex: ~(P V (Q & R))
The scope of “&” is (Q & R)
The scope of “V” is (P V (Q & R))
The scope of “~” is ~(P V (Q & R))
In this case, “~” is the main operator because its scope is the entire
formula
Note: if the main operator is “&”, it is called a conjunction. If the main
operator is a “~”, it is called a negation…etc.
Practice Examples:
Identify WFFs/ not WFFS
a. (R & M V P)
b. (Q  P)
c. ~(P 7 Q) R
d. ~~~~~~~~(P & Q)
e. (~(Q) & P)
f. ((P & Q) S) V (S (P & Q))
g. (((~Q V ~P) ~(~P R) > (~Q & ~S))
h. ~(~P V ~~~~~~Q)
i. ((P & Q) R S)
a. No, there are no brackets to specify the formula
b. Yes
c. No, missing brackets around the entire formula
d. Yes
e. No, sentence letters will never require brackets by themselves
f. No, missing bracket at the end
g. No, missing brackets at the end
h. Yes
i. No, there are no brackets around R and S to specify the formula
Put together complex formulas:
j. Either Sam came or Grace came
k. If either Sam or Rachel came, then Isabel did not come
l. Grace came if and only if Sam did not come
m. If Isabel came if and only if Cole came, then Sam and Grace came
n. If Sam came, then either Beck did not come or Jack did not come
o. Either Grace did not come, or if Same came, then Rachel came
p. Grace came, but Sam did not come if Jack came
q. If Grace did not come, then neither Olivia nor Rachel came.
r. Either Sam came if Grace came, or Rachel came if and only if John did not come.
find more resources at oneclass.com
find more resources at oneclass.com
j. (S V G)
k. ((S V R) ~I)
l. (G  ~S)
m. ((I  C) (S & G))
n. (S (~B v ~J))
o. (~G V (S R))
p. (G (J ~S))
q. ((G S) V (~J  R)) Note: order of formula does not matter in biconditionals
find more resources at oneclass.com
find more resources at oneclass.com