Post on 22-Jan-2016
description
Auto-Epistemic Logic
• Proposed by Moore (1985)
• Contemplates reflection on self knowledge (auto-epistemic)
• Allows for representing knowledge not just about the external world, but also about the knowledge I have of it
Syntax of AEL
• 1st Order Logic, plus the operator L (applied to formulas)
• L means “I know ”• Examples:
MScOnSW → L MScSW
(or L MScOnSW → MScOnSW)
young (X) L studies (X) → studies (X)
Meaning of AEL
• What do I know?– What I can derive (in all models)
• And what do I not know?– What I cannot derive
• But what can be derived depends on what I know– Add knowledge, then test
Semantics of AEL
• T* is an expansion of theory T iff
T* = Th(T{L : T* |= } {L : T* |≠ })
• Assuming the inference rule /L :
T* = CnAEL(T {L : T* |≠ })
• An AEL theory is always two-valued in L, that is, for every expansion:
| L T* L T*
Knowledge vs. Belief
• Belief is a weaker concept– For every formula, I know it or know it not– There may be formulas I do not believe in,
neither their contrary
• The Auto-Epistemic Logic of knowledge and belief (AELB), introduces also operator B – I believe in
AELB Example
• I rent a film if I believe I’m neither going to baseball nor football games
Bbaseball Bfootball → rent_filme• I don’t buy tickets if I don’t know I’m going to
baseball nor know I’m going to football L baseball L football → buy_tickets
• I’m going to football or baseballbaseball football
• I should not conclude that I rent a film, but do conclude I should not buy tickets
Axioms about beliefs
• Consistency Axiom
B• Normality Axiom
B(F → G) → (B F → B G)
• Necessitation rule
F
B F
Minimal models
• In what do I believe?– In that which belongs to all preferred models
• Which are the preferred models?– Those that, for one same set of beliefs, have a minimal
number of true things
• A model M is minimal iff there does not exist a smaller model N, coincident with M on B e Latoms
• When is true in all minimal models of T, we write T |=min
AELB expansions
• T* is a static expansion of T iff
T* = CnAELB(T {L : T* |≠ }
{B : T* |=min })
where CnAELB denotes closure using the
axioms of AELB plus necessitation for L
The special case of AEB
• Because of its properties, the case of theories without the knowledge operator is especially interesting
• Then, the definition of expansion becomes:
T* = (T*)
where (T*) = CnAEB(T {B : T* |=min })
and CnAEB denotes closure using the axioms of AEB
Least expansion
• Theorem: Operator is monotonic, i.e.
T T1 T2 → (T1) (T2)• Hence, there always exists a minimal
expansion of T, obtainable by transfinite induction:– T0 = Cn(T)
– Ti+1 = (Ti)
– T = U T (for limit ordinals )
Consequences
• Every AEB theory has at least one expansion
• If a theory is affirmative (i.e. all clauses have at least a positive literal) then it has at least a consistent expansion
• There is a procedure to compute the semantics
LP forKnowledge Representation
• Due to its declarative nature, LP has become a prime candidate for Knowledge Representation and Reasoning
• This has been more noticeable since its relations to other NMR formalisms were established
• For this usage of LP, a precise declarative semantics was in order
Language• A Normal Logic Programs P is a set of rules:
H A1, …, An, not B1, … not Bm (n,m 0)
where H, Ai and Bj are atoms
• Literal not Bj are called default literals
• When no rule in P has default literal, P is called definite
• The Herbrand base HP is the set of all instantiated atoms from program P.
• We will consider programs as possibly infinite sets of instantiated rules.
Declarative Programming
• A logic program can be an executable specification of a problem
member(X,[X|Y]).
member(X,[Y|L]) member(X,L).
• Easier to program, compact code• Adequate for building prototypes• Given efficient implementations, why not use it to
“program” directly?
LP and Deductive Databases
• In a database, tables are viewed as sets of facts:
• Other relations are represented with rules:
),(
).,(
londonlisbonflight
adamlisbonflight
LondonLisbon
AdamLisbon
tofromflight
).,(),(
).,(),,(),(
).,(),(
BAconnectionnotBAherchooseAnot
BCconnectionCAflightBAconnection
BAflightBAconnection
LP and Deductive DBs (cont)
• LP allows to store, besides relations, rules for deducing other relations
• Note that default negation cannot be classical negation in:
• A form of Closed World Assumption (CWA) is needed for inferring non-availability of connections
).,(),(
).,(),,(),(
).,(),(
BAconnectionnotBAherchooseAnot
BCconnectionCAflightBAconnection
BAflightBAconnection
Default Rules
• The representation of default rules, such as
“All birds fly”can be done via the non-monotonic operator not
).(
).(
).()(
).()(
.)(),()(
ppenguin
abird
PpenguinPabnormal
PpenguinPbird
AabnormalnotAbirdAflies
The need for a semantics
• In all the previous examples, classical logic is not an appropriate semantics– In the 1st, it does not derive not member(3,[1,2])
– In the 2nd, it never concludes choosing another company
– In the 3rd, all abnormalities must be expressed
• The precise definition of a declarative semantics for LPs is recognized as an important issue for its use in KRR.
2-valued Interpretations
• A 2-valued interpretation I of P is a subset of HP
– A is true in I (ie. I(A) = 1) iff A I– Otherwise, A is false in I (ie. I(A) = 0)
• Interpretations can be viewed as representing possible states of knowledge.
• If knowledge is incomplete, there might be in some states atoms that are neither true nor false
3-valued Interpretations
• A 3-valued interpretation I of P is a set
I = T U not F
where T and F are disjoint subsets of HP
– A is true in I iff A T– A is false in I iff A F– Otherwise, A is undefined (I(A) = 1/2)
• 2-valued interpretations are a special case, where:
HP = T U F
Models
• Models can be defined via an evaluation function Î:– For an atom A, Î(A) = I(A)– For a formula F, Î(not F) = 1 - Î(F)– For formulas F and G:
• Î((F,G)) = min(Î(F), Î(G))• Î(F G)= 1 if Î(F) Î(G), and = 0 otherwise
• I is a model of P iff, for all rule H B of P:
Î(H B) = 1
Minimal Models Semantics• The idea of this semantics is to minimize positive
information. What is implied as true by the program is true; everything else is false.
• {pr(c),pr(e),ph(s),ph(e),aM(c),aM(e)} is a model• Lack of information that cavaco is a physicist, should indicate that he isn’t• The minimal model is: {pr(c),ph(e),aM(e)}
)(
)(
)()(
cavacopresident
einsteinphysicist
XphysicistXaticianableMathem
Minimal Models Semantics
D[Truth ordering] For interpretations I and J, I J iff for all atom A, I(A) I(J), i.e.
TI TJ and FI FJ
T Every definite logic program has a least (truth ordering) model.
D[minimal models semantics] An atom A is true in (definite) P iff A belongs to its least model. Otherwise, A is false in P.
TP operator• The minimal models of a definite P can be
computed (bottom-up) via operator TP
D [TP] Let I be an interpretation of definite P.
TP(I) = {H: (H Body) P and Body I}
T If P is definite, TP is monotone and continuous. Its minimal fixpoint can be built by:
I0 = {} and In = TP(In-1)
T The least model of definite P is TP({})
On Minimal Models
• SLD can be used as a proof procedure for the minimal models semantics:– If the is a SLD-derivation for A, then A is true– Otherwise, A is false
• The semantics does not apply to normal programs:– p not q has two minimal models:
{p} and {q}
There is no least model !