fol formal logic - suraj @ lumssuraj.lums.edu.pk/~cs331a05/lecture4(fol).pdf · cs 331 dr m m awais...

CS 331 Dr M M Awais 1

FOL

Formal LogicFormal Logic• The most widely used formal logic method is

FIRST-ORDER PREDICATE LOGIC

Reference: Chapter Two The predicate CalculusLuger’s BookExamples included from Norvig and Russel.


FOL

First-order logic

• Whereas propositional logic assumes the world contains facts,

• first-order logic (like natural language) assumes the world contains– Objects: people, houses, numbers, colors, baseball

games, wars, …– Relations: red, round, prime, brother of, bigger than,

part of, comes between, …– Functions: father of, best friend, one more than, plus,

…


FOL

Syntax of FOL: Basic elements

• Constants john, 2, lums,... • Predicates brother, >,...• Functions sqrt, leftsideOf,...• Variables X,Y,A,B...• Connectives ¬, ⇒, ∧, ∨, ⇔• Equality = • Quantifiers ∀, ∃


FOL

Truth in first-order logic• Sentences are true with respect to a model and an

interpretation

• Model contains objects (domain elements) and relations among them

• Interpretation specifies referents forconstant symbols → objectspredicate symbols → relationsfunction symbols → functional relations

• An atomic sentence predicate(term1,...,termn) is trueiff the objects referred to by term,,..., term ,are in the relation referred to by predicate


FOL

AlphabetsAlphabets--IIPredicates, variables, functions,constants,

connectives, quantifiers, and delimiters

Constants: (first letter small)bLUE a colorsanTRO a carcrow a birdVariables: (first letter capital)Dog: an element that is a dog, but unspecifiedColor: an unspecified color


FOL

AlphabetsAlphabets--IIIIFunction:

Maps Sentences to Objects

Ali is father of Babar father(babar) = alifather_of(baber) = ali

• Interpretation has to be very clear.• If you write father(baber), the answer should be ali• For the above functions the arity is 1

(number of arguments to the function)


FOL

AlphabetsAlphabets--IIIIFunctions:1) shahid likes zahid likes(shahid) = zahid2) atif likes abid likes(atif) = abid3) Constants to Variables likes(X) = Y

{X,Y} have two possible BINDINGSBINDINGS

{X, Y} could be {shahid, zahid}Or{X,Y} could be {atif, abid}

likes(X) =Y

Substitutions:

For 1 to be true:{shahid/X, zahid/Y}

For 2 to be true:{atif/X, abid/Y}


FOL

AlphabetsAlphabets--IIIIPredicate

Maps Sentences to Truth Values (True/False)

1) Shahid is student student(shahid)2) Sana is a girl girl(sana)3) Father of baber is elder than Hamza

elder(father(babar), hamza)For 1 and 2 arity is 1 and for 3 the arity is 2


FOL

AlphabetsAlphabets--IIIIPredicate1) Shahid is a good studentstudent(shahid,good) or good_student(shahid)2) Sana is a friend of Saima, Sana and Saima

both are girls friend_of(sana,saima)^girl(sana)^girl(saima)3) Bill helps Fredhelps(bill,fred)


FOL

Atomic sentencesAtomic sentence = predicate (term1,...,termn)

or term1 = term2

term = function (term1,...,termn) or constantor variable

• brother(john,richard)• greater(length(leftsideOf(squareA)), length(leftsideOf(squareB)))• >(length(leftsideOf(squareA)), length(leftsideOf(squareB)))

Functions cannot be atomic sentences


FOL

AlphabetsAlphabets--IIIIIIConnectives:^ andv or~ not

ImplicationQuantificationAll persons can seeThere is a person who cannot see

Universal quantifiers ∀ (ALL)Existential quantifiers ∃ (There exists)


FOL

Complex sentences• Complex sentences are made from atomic

sentences using connectives¬S, S1∧ S2, S1 ∨ S2, S1⇒ S2, S1⇔ S2,

sibling(ali,hamza) ⇒ sibling(hamza,ali)>(1,2) ∨ ≤ (1,2) (1 is greater than 2 or less than equal to 2)>(1,2) ∧ ¬ >(1,2) (1 is greater than 2 and is not greater than

equal to 2)


FOL

ExamplesExamplesMy house is a blue, two-story, with red shutters, and is a

corner houseblue(my-house)^two-story(my-house)^red-shutters(my-

house)^corner(my-house)

Ali bought a scooter or a carbought(ali , car) v bought(ali , scooter)

IF fuel, air and spark are present the fuel will combustpresent(spark)^present(fuel)^present(air)

combustion(fuel)


FOL

Universal quantification• ∀<variables> <sentence>Everyone at LUMS is smart:

∀X at(X, lums) ⇒ smart(X)

• ∀X P is true in a model m iff P is true with X being each possible object in the model�

• Roughly speaking, equivalent to the conjunction of instantiations of P�

at(rabia,lums) ⇒ smart(rabia) ∧ at(shahid,lums) ⇒ smart(shahid) ∧ at(lums,lums) ⇒ smart(lums)∧ ...


FOL ExamplesExamplesAll people need air

∀X[person(X) need_AIR(X)]

The owner of the car also owns the boat

[owner(X , car) ^ car(X , boat)]

Formulate the following expression in the PC:“Ali is a computer science student but not a pilot or

a football player”

cs_STUDENT(ali) ∧ (¬ pilot(ali) ∨ ¬ ft_PLAYER(ali) )


FOL

ExamplesExamples

Restate the sentence in the following way:1. Ali is a computer science (CS) student2. Ali is not a pilot3. Ali is not a football player

cs_student(ali)^~pilot(ali)^

~football_player(ali)


FOL

ExamplesExamplesStudying fuzzy systems is exciting and applying logic is great

fun if you are not going to spend all of your time slaving over the terminal

∀ X(~slave_terminal(X) [fs_eciting(X)^logic_fun(X)])

Every voter either favors the amendment or despises it

∀X[voter(X) [favor(X , amendment) v despise(X,amendment)]

^ ~[favor(X , amendment) v despise(X , amendment)]

(this part simply endorses the statement, may not be required)


FOL

Undecidable Predicate

• For which exhaustive testing is required• Example:∀X likes(zahra, X)

• This sentence is computationally impossible to calculate

• Scope of problem domain is to be limited to remove this problem, – i.e., X is a variable representing final year female

student in the AI class, compared to an X representing all the people in the city of Lahore


FOL

Robotic Arm Example

•Represent the initial details of the system•Generate sentences of descriptive and or implicative nature•Modify the facts using new sentences

a

b

c

d


FOL

Example: Robotic Arm

• Represent the initial details of the systems

a

b

c

d

on(a,b)on( c,d)ontable(b)ontable(d)clear(a)clear(c)hand_empty


FOL

Goal:To pick a block and place it over another block

Define predicate: stack_on(X,Y)General sentence:

Conditions ConclusionsWhat could the conditions?

hand_emptyclear (X)clear (Y)pick (X)put_on (X,Y)

a

b

c

d

hand_empty ^ clear (X) ^ clear (Y) ^ pick (X) ^ put_on (X,Y) stack_on (X,Y)


FOL

Goal:To pick a block and place it over another block

hand_empty ^ clear (X) ^ clear (Y) ^ pick (X) ^ put_on (X,Y) → stack_on (X,Y)

hand_empty ^ clear (X) → pick (X)

clear(Y) ^ pick (X)→ put_on (X,Y)

put_on (X,Y) → stack_on (X,Y)

Semantically more correct

hand_empty could be written as empty(hand), if hand_empty is in the knowledge base, then hand is empty otherwise false.

put_on (X,Y) → stack_on (X,Y)is in fact equivalence


FOL

Example: Robotic Arm• Modify details of the systems

a

b

c

d

on(b,a)on( c,d)ontable(b)ontable(d)clear(a)clear(c)hand_empty

a

b

c

d

on(b,a)on( c,d)on(e,a)ontable(b)ontable(d)clear(c)clear(e)hand_empty

e


FOL

Models for FOL: Example


FOL

A common mistake to avoid• Represent: Everyone at LUMS is smart∀X at(X,lums) ∧ smart(X)∀X at(X, lums) ⇒ smart(X)• Common mistake:

using ∧ as the main connective with ∀: means“Everyone is at LUMS and everyone is smart”“Everyone at LUMS is smart” • Typically, • ⇒ is the main connective with ∀


FOL

Existential quantification• ∃<variables> <sentence>

• Someone at LUMS is smart:• ∃X at(X,lums) ∧ smart(X)

• ∃X P is true in a model m iff P is true with X being some possible object in the model

• Roughly speaking, equivalent to the disjunction of instantiations of P

at(sana,lums) ∧ smart(sana) ∨ at(bashir,lums) ∧ smart(bashir) ∨ at(lums,lums) ∧ smart(lums)∨ ...


FOL

Another common mistake to avoid

• Typically, ∧ is the main connective with ∃

• Common mistake: using ⇒ as the main connective with ∃:

∃X at(X,lums) ⇒ smart(X)is true if there is anyone who is not at LUMS!

• Even if the antecedent is false the sentence can still be true (see the truth table of implication).


FOL

Properties of quantifiers• ∀X ∀Y is the same as ∀Y ∀X• ∃X ∃Y is the same as ∃Y ∃X

• ∃X ∀Y is not the same as ∀Y ∃X• ∃X ∀Y loves(X,Y)

“There is a person who loves everyone in the world”• ∀Y ∃X Loves(X,Y)

“Everyone in the world is loved by at least one person”

• Quantifier duality: each can be expressed using the other• ∀X likes(X,car) ¬∃X ¬likes(X,car)• ∃X likes(X,bread) ¬∀X ¬likes(X,bread)


FOL

Equality

• term1 = term2 is true under a given interpretation if and only if term1 and term2 refer to the same object

• Sibling in terms of Parent:∀X,Y sibling(X,Y)⇔[¬(X = Y) ∧ ∃M,F ¬ (M = F) ∧parent(M,X) ∧ parent(F,X) ∧parent(M,Y) ∧ parent(F,Y)]


FOL

Using FOL

The kinship domain:• Brothers are siblings∀X,Y brother(X,Y) ⇔ sibling(X,Y)

• One's mother is one's female parent∀M,C mother(C) = M ⇔ (female(M) ∧parent(M,C))

• “Sibling” is symmetric∀X,Y sibling(X,Y) ⇔ sibling(Y,X)


FOL

Rules: Wumpus world

• Perception– ∀T,S,B percept([S,B,glitter],T) ⇒ glitter(T)�

• Reflex– ∀T glitter(T) ⇒ bestAction(grab,T)


FOL

Deducing Squares/PropertiesWhat are Adjacent Squares∀X,Y,A,B adjacent([X,Y],[A,B]) ⇔[A,B] ∈ {[X+1,Y], [X-1,Y],[X,Y+1],[X,Y-1]}

Properties of squares:∀S,T at(agent,S,T) ∧ breeze(T) ⇒ breezy(S)

Squares are breezy near a pit:Diagnostic rule---infer cause from effect∀S breezy(S) ⇒ adjacent(R,S) ∧ pit(R)

Causal rule---infer effect from cause∀R pit(R) ⇒ [∀S adjacent(R,S) ⇒ breezy(S)]


FOL

Knowledge engineering in FOL1. Identify the task�2. Assemble the relevant knowledge�3. Decide on a vocabulary of predicates, functions,

and constants�4. Encode general knowledge about the domain�5. Encode a description of the specific problem

instance�6. Pose queries to the inference procedure and get

answers�7. Debug the knowledge base�


FOL

The electronic circuits domain

One-bit full adder�


FOL

The electronic circuits domain1. Identify the task�

– Does the circuit actually add properly? (circuit verification)�

2. Assemble the relevant knowledge�– Composed of wires and gates; Types of gates (AND,

OR, XOR, NOT)�– Irrelevant: size, shape, color, cost of gates�

3. Decide on a vocabulary�– Alternatives:�

type(x1) = xor�type(x1, xor)xor(x1)�


FOL

The electronic circuits domain4. Encode general knowledge of the domain�

– ∀T1,T2 connected(T1, T2) ⇒ signal(T1) = signal(T2)– ∀T signal(T) = 1 ∨ signal(T) = 0�– 1 ≠ 0�– ∀T1,T2 connected(T1, T2) ⇒ connected(T2, T1)�– ∀G type(G) = OR ⇒ signal(out(1,G)) = 1 ⇔ ∃N

signal(in(N,G)) = 1�– ∀G type(G) = AND ⇒ signal(out(1,G)) = 0 ⇔ ∃N

signal(in(N,G)) = 0�– ∀G type(G) = XOR ⇒ signal(out(1,G)) = 1 ⇔

signal(in(1,G)) ≠ signal(in(2,G))�– ∀G type(G) = NOT ⇒ signal(out(1,G)) ≠

signal(in(1,G))�


FOL

The electronic circuits domain5. Encode the specific problem instance�

type(x1) = xor type(x2) = xortype(a1) = and type(a2) = andtype(o1) = or

connected(out(1,x1),in(1,x2)) connected(in(1,c1),in(1,x1))connected(out(1,x1),in(2,a2)) connected(in(1,c1),in(1,a1))connected(out(1,o2),in(1,o1)) connected(in(2,c1),in(2,x1))connected(out(1,a1),in(2,o1)) connected(in(2,c1),in(2,a1))connected(out(1,x2),out(1,c1)) connected(in(3,c1),in(2,x2))connected(out(1,o1),out(2,c1)) connected(in(3,c1),in(1,a2))�


FOL

The electronic circuits domain

6. Pose queries to the inference procedure�What are the possible sets of values of all the

terminals for the adder circuit? �∃I1,I2,I3,O1,O2 signal(in(1,c_1)) = I1 ∧ signal(in(2,c1)) = I2 ∧ signal(in(3,c1)) = I3 ∧ signal(out(1,c1)) = O1 ∧signal(out(2,o1)) = O2�

7. Debug the knowledge base�May have omitted assertions like 1 ≠ 0�


FOL

Summary

• First-order logic:�– objects and relations are semantic primitives– syntax: constants, functions, predicates,

equality, quantifiers�

• Increased expressive power: sufficient to define wumpus world �


FOL

Operations• Unification: Algorithm for determining the

subitutions needed to make two predicate calculus expressions match

• Skolemization: A method of removing or replacing existential quantifiers

• Composition: If S and S` are two substitutions sets, then the composition of S and S` (SS`) is obtained by applying the elements of S to the elements of S` and finally adding the results

fol formal logic - suraj @ lumssuraj.lums.edu.pk/~cs331a05/lecture4(fol).pdf · cs 331 dr m m awais...

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