computer science cpsc 322 lecture 22 logical consequences, proof procedures (ch 5.2.2)
TRANSCRIPT
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Computer Science CPSC 322
Lecture 22
Logical Consequences,
Proof Procedures
(Ch 5.2.2)
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Announcements
• Practice Exercise on Planning as CSP (can help with Assign 3 Q2
• Practice exercise on logic is available on the class syllabus
Pay special attention to “Directed Questions”. They can really help you do well on the conceptual questions
part of the finalShow you how to be precise when you define concepts
• After today you can do Q3a in assign 3
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Lecture Overview
• Recap Lecture 21• Logical Consequences • Proof Procedures
• Bottom-Up Proof Procedure
Slide 3
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Where Are We?Environment
Problem Type
Query
Planning
Deterministic Stochastic
Constraint Satisfaction Search
Arc Consistency
Search
Search
Logics
STRIPS
Vars + Constraints
Value Iteration
Variable
Elimination
Belief Nets
Decision Nets
Markov Processes
Static
Sequential
Representation
ReasoningTechnique
Variable
EliminationBack to static
problems, but with richer
representation Slide 4
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Logic: a framework for representation & reasoning
• When we represent a domain about which we have only partial (but certain) information, we need to represent….• Objects, properties, sets, groups, actions, events, time, space, …
• All these can be represented as • Objects• Relationships between objects
• Logic is the language to express knowledge about the world this way
5
We will start with a simple logicPrimitive elements are propositions: Boolean variables that can be {true, false}
Two kinds of statements:that a proposition is truethat a proposition is true if one or more other propositions are true
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To Define a Logic We Need
• Syntax: specifies the symbols used, and how they can be combined to form legal sentences • Knowledge base is a set of sentences in the language
• Semantics: specifies the meaning of symbols and sentences
• Reasoning theory or proof procedure: a specification of how an answer can be produced.• Sound: only generates correct answers with respect to
the semantics
• Complete: Guaranteed to find an answer if it exists
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Propositional Definite Clauses: Syntax
Definition (atom)
An atom is a symbol starting with a lower case letter
Definition (body)
A body is an atom or is of the form b1 ∧ b2 where b1 and b2 are bodies.
Definition (definite clause)
A definite clause is
- an atom or
- a rule of the form h ← b where h is an
atom (“head”) and b is a body. (Read this as “h if b”.)
Definition (KB)
A knowledge base (KB) is a set of definite clauses
Examples: p1; live_l1
Examples: p1 ∧ p2; ok_w1 ∧ live_w0
Examples: p1 ← p2; live_w0 ← live_w1 ∧ up_s2
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Propositional Definite Clauses: SemanticsDefinition (interpretation)
An interpretation I assigns a truth value to each atom.
Definition (truth values of statements)• A body b1 ∧ b2 is true in I if and only if b1 is true in
I and b2 is true in I.• A rule h ← b is false in I if and only if b is true in I
and h is false in I.• A knowledge base KB is true in I if and only if
every clause in KB is true in I.
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PDC Semantics: Knowledge Base (KB)
p q r s
I1 true true false false
prs ← q p∧
pqs ← q
pq ← r s∧
A. KB1 B. KB2C. KB3
Which of the three KB above is True in I1
• A knowledge base KB is true in I if and only if every clause in KB is true in I.
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Slide 10
PDC Semantics: Knowledge Base (KB)
p q r s
I1 true true false false
pq ← r s∧
KB3
Which of the three KB above are True in I1 ?KB3
• A knowledge base KB is true in I if and only if every clause in KB is true in I.
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Propositional Definite Clauses: Semantics
Definition (model)A model of a knowledge base KB is an interpretation in
which KB is true.
Similar to CSPs: a model of a set of clauses is an interpretation that makes all of the clauses true
Definition (interpretation)
An interpretation I assigns a truth value to each atom.
Definition (truth values of statements)• A body b1 ∧ b2 is true in I if and only if b1 is true in
I and b2 is true in I.• A rule h ← b is false in I if and only if b is true in I
and h is false in I.• A knowledge base KB is true in I if and only if
every clause in KB is true in I.
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PDC Semantics: Example for models
p ← q
KB = q
r ← s
Definition (model)A model of a knowledge base KB is an interpretation in
which every clause in KB is true.
p q r s
I1 T T T T
I2 F F F F
I3 T T F F
I4 T T T F
I5 F T F T
Which of the interpretations below are models of KB?
B. I1 , I3
C. I1, I3, I4
D. All of them
A. I3
Slide 12
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PDC Semantics: Example for models
p q r s p ← q q r ← s KB
I1 T T T T T T T T
I2 F F F F T F T F
I3 T T F F T T T T
I4 T T T F T T T T
I5 F T F T F T F F
Definition (model)A model of a knowledge base KB is an interpretation in
which every clause in KB is true.
p ← q
KB = q
r ← s
Which of the interpretations below are models of KB? All interpretations where KB is true: I1, I3, and I4
Slide 13
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Slide 14
OK but….…. Who cares? Where are we going with this?
Remember what we want to do with Logic
1) Tell the system knowledge about a task domain.• This is your KB• which expresses true statements about the world
2) Ask the system whether new statements about the domain are true or false.• We want the system responses to be
– Sound: only generates correct answers with respect to the semantics– Complete: Guaranteed to find an answer if it exists
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For Instance
1) Tell the system knowledge about a task domain.
2) Ask the system whether new statements about the domain are true or false
• live_w4? lit_l2?
Slide 15
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Slide 16
To Obtain This We Need One More Definition
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Lecture Overview
• Recap Lecture 21• Logical Consequences • Proof Procedures
• Bottom-Up Proof Procedure
Slide 17
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Slide 18
To Obtain This We Need One More Definition
Definition (logical consequence)If KB is a set of clauses and G is a conjunction of atoms, G is a logical consequence of KB, written KB ⊧ G, if G is true in every model of KB.
• we also say that G logically follows from KB, or that KB entails G.• In other words, KB ⊧ G if there is no interpretation in which KB is true and G is false.
• when KB is TRUE, then G must be TRUE
• We want a reasoning procedure that can find all and only the logical consequences of a knowledge base
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.
.
q
rqpKB
Example of Logic Entailment
How many models are there?
A. 1 B. 2 C. 3 D. 4
E. 5
Slide 19
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.
.
q
rqpKB
Interpretationsr q p
T T T
T T F
T F T
T F F
F T T
F T F
F F T
F F F
Example of Logic Entailment
How many models are there?
A. 1 B. 2 C. 3 D. 4
E. 5
Slide 20
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.
.
q
rqpKB
Interpretationsr q p
T T T
T T F
T F T
T F F
F T T
F T F
F F T
F F F
Models
Which atoms are logically entailed? q
Example of Logic Entailment
• We want a reasoning procedure that can find all and only the logical consequences of a knowledge base Slide 21
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Example: Logical Consequences
.
.
.
sr
q
qp
KB
p q r s
I1 true true true true
I2 true true true false
I3 true true false false
I4 true true false true
I5 false true true true
I6 false true true false
I7 false true false false
I8 false true false true
I9 true false true true
I10 true false true false
I11 true false false false
I12 true false false false
….. …… ….. …… …….
Which of the following is true?
A. KB ⊧ q and KB ⊧ rB. KB ⊧ q, and KB ⊧ s
C. KB ⊧ q, and KB ⊧ pD. KB ⊧ r, KB ⊧ s,
E. None of the above
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Example: Logical Consequences
.
.
.
sr
q
qp
KB
p q r s
I1 true true true true
I2 true true true false
I3 true true false false
I4 true true false true
I5 false true true true
I6 false true true false
I7 false true false false
I8 false true false true
I9 true false true true
I10 true false true false
I11 true false false false
I12 true false false false
….. …… ….. …… …….
Which of the following is true?
• KB ⊧ q,
• KB ⊧ p,
• KB ⊧ s,
• KB ⊧ r
Slide 23
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Example: Logical Consequences
.
.
.
sr
q
qp
KB
p q r s
I1 true true true true
I2 true true true false
I3 true true false false
I4 true true false true
I5 false true true true
I6 false true true false
I7 false true false false
I8 false true false true
I9 true false true true
I10 true false true false
I11 true false false false
I12 true false false false
….. …… ….. …… …….
Which of the following is true?
• KB ⊧ q,
• KB ⊧ p,
• KB ⊧ s,
• KB ⊧ r
T
T
F
FSlide 24
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Example: Logical Consequences
.
.
.
sr
q
qp
KB
p q r s
I1 true true true true
I2 true true true false
I3 true true false false
I4 true true false true
I5 false true true true
I6 false true true false
I7 false true false false
I8 false true false true
I9 true false true true
I10 true false true false
I11 true false false false
I12 true false false false
….. …… ….. …… …….
Which of the following is true?
C. KB ⊧ q, and KB ⊧ p
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User’s View of Semantics
• Choose a task domain: intended interpretation.
• For each proposition you want to represent, associate a proposition symbol in the language.
• Tell the system clauses that are true in the intended interpretation: axiomatize the domain.
• Ask questions about the intended interpretation.
• If KB |= g , then g must be true in the intended interpretation.
Slide 26
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Computer’s View of Semantics
• The computer doesn’t have access to the intended interpretation.
• All it knows is the knowledge base.
• The computer can determine if a formula is a logical consequence of KB.
• If KB |= g then g must be true in the intended interpretation.
• Otherwise, there is a model of KB in which g is false. This could be the intended interpretation.
The computer wouldn't know!
Slide 27
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Computer’s View of Semantics
• Otherwise, there is a model of KB in which g is false. This could be the intended interpretation.
The computer wouldn't know
.
.
.
sr
q
qp
KBp q r s
I1 true true true true
I2 true true true false
Slide 28
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To Define a Logic We Need
• Syntax: specifies the symbols used, and how they can be combined to form legal sentences • Knowledge base is a set of sentences in the language
• Semantics: specifies the meaning of symbols and sentences
• Reasoning theory or proof procedure: a specification of how an answer can be produced (sound and complete)• Bottom-up and Top-Down Proof Procedure for Finding
Logical Consequence
Slide 29
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Lecture Overview
• Recap Lecture 21• Logical Consequences • Proof Procedures
• Bottom-Up Proof Procedure
Slide 30
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Proof Procedures• A proof procedure is a mechanically derivable demonstration that
a formula logically follows from a knowledge base.• Given a proof procedure P, KB ⊦ P g means g can be derived
from knowledge base KB with the proof procedure.
• If I tell you I have a proof procedure for PDCL• What do I need to show you in order for you to trust my
procedure?
That is sound and complete
Slide 31
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Soundness and Completeness
• Completeness of proof procedure P: need to prove that
If g is true in all models of KB (KB g)⊧ then g is derived by the procedure (KB ⊦P g)
Definition (completeness)
A proof procedure P is complete if KB g implies KB ⊧ ⊦P g.
complete: every atom that logically follows from KB is derived by P
• Soundness of proof procedure P: need to prove that
Definition (soundness)
A proof procedure P is sound if KB ⊦P g implies KB g.⊧
If g can be derived by the procedure (KB ⊦P g) then g is true in all models of KB (KB g)⊧
sound: every atom derived by P follows logically from KB (i.e. is true in every model)
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Simple Proof Procedure
problem with this approach?
• If there are n propositions in the KB, must check all the interpretations!
Goal of proof theory • find sound and complete proof procedures that allow us to
prove that a logical formula follows from a KB avoiding to do the above
Simple proof procedure S• Enumerate all interpretations• For each interpretation I, check whether it is a model of KB
i.e., check whether all clauses in KB are true in I
• KB ⊦S g if g holds in all such models
Slide 33
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Lecture Overview
• Recap Lecture 21• Logical Consequences • Proof Procedures
• Bottom-Up Proof Procedure
Slide 35
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Bottom-up proof procedure
• One rule of derivation, a generalized form of modus ponens:
• If “h ← b1 … bm" is a clause in the knowledge base, and
each bi has been derived,
then h can be derived.
• This rule also covers the case when m = 0.
Slide 36
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Bottom-up proof procedure
C :={};repeat
select clause “h ← b1 ∧ … ∧ bm” in KB such that bi ∈ C for all i, and h ∉ C;
C := C ∪ { h }until no more clauses can be selected.
KB ⊦ G if G ⊆ C at the end of this procedure
Slide 37
The C at the end of BU procedure is a fixed point:• Further applications of our rule of derivation will not
change C!
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C := {};
repeat
select clause h ← b1 … bm in KB such that bi C for all i, and h C;
C := C {h}
until no more clauses can be selected.
Bottom-up proof procedure: example
a ← b c
a ← e f
b ← f k
c ← e
d ← k
e.
f ← j e
f ← c
j ← c
{}
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C := {};
repeat
select clause h ← b1 … bm in KB such that bi C for all i, and h C;
C := C {h}
until no more clauses can be selected.
Bottom-up proof procedure: example
a ← b c
a ← e f
b ← f k
c ← e
d ← k
e.
f ← j e
f ← c
j ← c
{}
{e}
{c,e}
{c,e,f}
{c,e,f,j}
{c,e,f,j, a}
Done.Slide 39
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Bottom-up proof procedure: Example
z ← f ∧ e
q ← f ∧ g ∧ z
e ← a ∧ babrf A. KB ⊦ {z , q , a}
KBC := {};
repeat
select clause h ← b1 … bm in KB such that bi C for all i, and h C;
C := C {h}
until no more clauses can be selected.
Which of the following is true?
B. KB ⊦ {r , z , b}
C. KB ⊦ {q,a}
Slide 40
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Bottom-up proof procedure: Example
z ← f ∧ e
q ← f ∧ g ∧ z
e ← a ∧ babrf
KBC := {};
repeat
select clause h ← b1 … bm in KB such that bi C for all i, and h C;
C := C {h}
until no more clauses can be selected.
Which of the following is true?
B. KB ⊦ {r , z , b}
Slide 41
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Learning Goals Up To Here
• PDCL syntax & semantics- Verify whether a logical statement belongs to the language of
propositional definite clauses
‾ Verify whether an interpretation is a model of a PDCL KB. ‾ Verify when a conjunction of atoms is a logical consequence
of a knowledge base
• Bottom-up proof procedure• Define/read/write/trace/debug the Bottom Up (BU) proof
procedure
Next Time: we will prove soundness and completenessSlide 42