Transcript Possibility Theory and its applications: a retrospective and

Possibility Theory and its
applications: a retrospective and
prospective view
D. Dubois, H. Prade
IRIT-CNRS, Université Paul Sabatier
31062 TOULOUSE FRANCE
Outline
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Basic definitions
Pioneers
Qualitative possibility theory
Quantitative possibility theory
Possibility theory is an uncertainty theory
devoted to the handling of
incomplete information.
• similar to probability theory because it is based on setfunctions.
• differs by the use of a pair of dual set functions
(possibility and necessity measures) instead of only one.
• it is not additive and makes sense on ordinal structures.
The name "Theory of Possibility" was coined by Zadeh in 1978
The concept of possibility
• Feasibility: It is possible to do something
(physical)
• Plausibility: It is possible that something occurs
(epistemic)
• Consistency : Compatible with what is known
(logical)
• Permission: It is allowed to do something
(deontic)
POSSIBILITY DISTRIBUTIONS
(uncertainty)
• S: frame of discernment (set of "states of the world")
• x : ill-known description of the current state of affairs
taking its value on S
• L: Plausibility scale: totally ordered set of plausibility
levels ([0,1], finite chain, integers,...)
• A possibility distribution πx attached to x is a mapping from
S to L : s, πx(s)  L, such that s, πx(s) = 1
(normalization)
• Conventions:
πx(s) = 0 iff x = s is impossible, totally excluded
πx(s) = 1 iff x = s is normal, fully plausible, unsurprizing
EXAMPLE : x = AGE OF PRESIDENT
• If I do not know the age of the president, I may
have statistics on presidents ages… but generally
not, or they may be irrelevant.
• partial ignorance :
–
70 ≤ x ≤ 80 (sets, intervals)
a uniform possibility distribution
π(x) = 1
x  [70, 80]
=0
otherwise
• partial ignorance with preferences : May have
reasons to believe that 72 > 71  73 > 70  74 >
75 > 76 > 77
EXAMPLE : x = AGE OF PRESIDENT
• Linguistic information described by fuzzy
sets: “ he is old ” : π = µOLD
• If I bet on president's age: I may come
up with a subjective probability !
But this result is enforced by the setting of
exchangeable bets (Dutch book argument).
Actual information is often poorer.
A possibility distribution is the representation of a state of
knowledge:
a description of how we think the state of affairs is.
• π' more specific than π in the wide sense
if and only if π' ≤ π
In other words: any value possible for π' should be at least as
possible for π
that is, π' is more informative than π
• COMPLETE KNOWLEDGE : The most specific ones
• π(s0) = 1 ;
π(s) = 0 otherwise
• IGNORANCE : π(s) = 1,  s  S
POSSIBILITY AND NECESSITY
OF AN EVENT
• A possibility distribution on S (the normal values of x)
• an event A
How confident are we that x  A  S ?
(A) = maxuA π(s);
The degree of possibility that x  A
N(A) = 1 – (Ac)=min uA 1 – π(s)
The degree of certainty (necessity) that x  A
Comparing the value of a quantity x to a threshold
when the value of x is only known to belong to an
interval [a, b].
• In this example, the available knowledge is
modeled by p(x) = 1 if x  [a, b], 0 otherwise.
• Proposition p = "x > " to be checked
• i) a > : then x >  is certainly true :
N(x >  ) = (x >  ) = 1.
• ii) b < : then x >  is certainly false ;
N(x >  ) = (x >  ) = 0.
• iii) a ≤  ≤ b: then x >  is possibly true or false;
N(x >  ) = 0; (x >  ) = 1.
Basic properties
(A) = to what extent at least one element in A is
consistent with π (= possible)
N(A) = 1 – (Ac) = to what extent no element outside A is
possible = to what extent π implies A
(A  B) = max((A), (B)); N(A  B) = min(N(A),
N(B)).
Mind that most of the time :
(A  B) < min((A), (B));
N(A B) > max(N(A), N(B)
Corollary N(A) > 0  (A) = 1
Pioneers of possibility theory
• In the 1950’s, G.L.S. Shackle called "degree of
potential surprize" of an event its degree of impossibility.
• Potential surprize is valued on a disbelief scale, namely a
positive interval of the form [0, y*], where y* denotes the
absolute rejection of the event to which it is assigned.
• The degree of surprize of an event is the degree of surprize
of its least surprizing realization.
• He introduces a notion of conditional possibility
Pioneers of possibility theory
• In his 1973 book, the philosopher
David Lewis
considers a relation between possible worlds he
calls "comparative possibility".
• He relates this concept of possibility to a notion of
similarity between possible worlds for defining the
truth conditions of counterfactual statements.
• for events A, B, C, A  B C  A  C  B.
• The ones and only ordinal counterparts to
possibility measures
Pioneers of possibility theory
• The philosopher L. J. Cohen considered the problem of
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legal reasoning (1977).
"Baconian probabilities" understood as degrees of
provability.
It is hard to prove someone guilty at the court of law by
means of pure statistical arguments.
A hypothesis and its negation cannot both have positive
"provability"
Such degrees of provability coincide with necessity
measures.
Pioneers of possibility theory
• Zadeh (1978) proposed an interpretation of membership
functions of fuzzy sets as possibility distributions encoding
flexible constraints induced by natural language
statements.
• relationship between possibility and probability: what is
probable must preliminarily be possible.
• refers to the idea of graded feasibility ("degrees of ease")
rather than to the epistemic notion of plausibility.
• the key axiom of "maxitivity" for possibility measures is
highlighted (also for fuzzy events).
Qualitative vs. quantitative possibility theories
• Qualitative:
– comparative: A complete pre-ordering ≥π on U
A well-ordered partition of U: E1 > E2 > … > En
– absolute: πx(s)  L = finite chain, complete lattice...
• Quantitative: πx(s)  [0, 1], integers...
One must indicate where the numbers come from.
All theories agree on the fundamental maxitivity axiom
(A  B) = max((A), (B))
Theories diverge on the conditioning operation
Ordinal possibilistic conditioning
• A Bayesian-like equation:
A) = min(A), 
A) is the maximal solution to this equation.
(B | A)
= 1 if A, B ≠ Ø, (A) = (A  B) > 0
= (A  B) if (A) > (A  B)
N(B | A) = 1 – (Bc| A)
• Independence
(B | A) = (B) implies A) = min(), 
Not the converse!!!!
QUALITATIVE POSSIBILISTIC REASONING
• The set of states of affairs is partitioned via π into a totally
ordered set of clusters of equally plausible states
E1 (normal worlds) > E2 >... En+1 (impossible worlds)
• ASSUMPTION: the current situation is normal.
By default the state of affairs is in E1
• N(A) > 0 iff (A) > (Ac)
iff A is true in all the normal situations
Then, A is accepted as an expected truth
• Accepted events are closed under deduction
A CALCULUS OF PLAUSIBLE INFERENCE
(B) ≥ (C) means « Comparing propositions on the
basis of their most normal models »
• ASSUMPTION for computing (B): the current situation
is the most normal where B is true.
• PLAUSIBLE REASONING = “ reasoning as if the current
situation were normal” and jumping to accepted
conclusions obtained from the normality assumption.
• DIFFERENT FROM PROBABILISTIC
REASONING BASED ON AVERAGING
ACCEPTANCE IS DEFEASIBLE
• If B is learned to be true, then the normal
situations become the most plausible ones in B,
and the accepted beliefs are revised accordingly
• Accepting A in the context where B is true:
(AB) > (Ac B) iff N(A | B) > 0
(conditioning)
• One may have
N(A) > 0 , N(Ac | B) > 0 :
non-monotony
PLAUSIBLE INFERENCE
WITH A POSSIBILITY DISTRIBUTION
Given a non-dogmatic possibility distribution π on S
(π(s) > 0, s)
Propositions A, and B
• A |=π B iff (A  B) > (A Bc)
It means that
B is true in the most plausible worlds where A is true
• This is a form of inference first proposed by
Shoham in nonmonotonic reasoning
PLAUSIBLE INFERENCE
WITH A POSSIBILITY DISTRIBUTION
A
B
š preferred worlds
(in A)
Example (continued)
• Pieces of knowledge like ∆ = {b f, p  b, p  ¬f}
can be expressed by constraints
(b  f) > ( b ¬f)
(p  b) > (p  ¬b)
(p  ¬f) > (p  f)
• the minimally specific π* ranks normal situations first:
¬p  b  f, ¬p ¬b
• then abnormal situations: ¬f  b
• Last, totally absurd situations f  p , ¬b p
Example (back to possibilistic logic)
 =material implication
• Ranking of rules: b f has less priority
that others according to p*:
N*(b f ) = N*(p  b) > N*(b f)
• Possibilistic base :
K = {(b f ), (p  b), (p  ¬f)},
with  < 
Applications of qualitative
possibility theory
• Exception-tolerant Reasoning in rule bases
• Belief revision and inconsistency handling in
deductive knowledge bases
• Handling priority in constraint-based reasoning
• Decision-making under uncertainty with
qualitative criteria (scheduling)
• Abductive reasoning for diagnosis under poor
causal knowledge (satellite faults, car engine testbenches)
ABSOLUTE APPROACH
TO QUALITATIVE DECISION
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A set of states S;
A set of consequences X.
A decision = a mapping f from S to X
f(s) is the consequence of decision f when the
state is known to be s.
• Problem : rank-order the set of decisions in XS
when the state is ill-known and there is a utility
function on X.
• This is SAVAGE framework.
ABSOLUTE APPROACH
TO QUALITATIVE DECISION
• Uncertainty on states is possibilistic
a function π: S  L
L is a totally ordered plausibility scale
• Preference on consequences:
a qualitative utility function µ: X  U
– µ(x) = 0
– µ(y) > µ(x)
– µ(x) = 1
totally rejected consequence
y preferred to x
preferred consequence
Possibilistic decision criteria
• Qualitative pessimistic utility (Whalen):
UPES(f) = minsS max(n(π(s)), µ(f(s)))
where n is the order-reversing map of V
– Low utility : plausible state with bad consequences
• Qualitative optimistic utility (Yager):
UOPT(f) = maxsS min(π(s), µ(f(s)))
– High utility: plausible states with good
consequences
The pessimistic and optimistic utilities are well-known fuzzy
pattern-matching indices
• in fuzzy expert systems:
– µ = membership function of rule condition
– π = imprecision of input fact
• in fuzzy databases
– µ = membership function of query
– π = distribution of stored imprecise data
• in pattern recognition
– µ = membership function of attribute template
– π = distribution of an ill-known object attribute
Assumption: plausibility and preference scales L and U are
commensurate
• There exists a common scale V that contains both L and U,
so that confidence and uncertainty levels can be compared.
– (certainty equivalent of a lottery)
• If only a subset E of plausible states is known
– π = E
– UPES(f) = minsE µ(f(s)) (utility of the worst
consequence in E)
criterion of Wald under ignorance
– UOPT(f)= maxsE µ(f(s))
On a linear state space
š
µo f
u*
u*
S
optimistic
prévision
pessimistic
prevision
Pessimistic qualitative utility of binary acts
xAy, with µ(x) > µ(y):
• xAy (s) = x if A occurs
= y if its complement Ac occurs
UPES(xAy) = median {µ(x), N(A), µ(y)}
• Interpretation: If the agent is sure enough of A, it is as if
the consequence is x: UPES(f) = µF(x)
If he is not sure about A it is as if the consequence is y:
UPES(f) = µF(y)
Otherwise, utility reflects certainty: UPES(f) = N(A)
• WITH UOPT(f) : replace N(A) by (A)
Representation theorem
for pessimistic possibilistic criteria
• Suppose the preference relation a on acts obeys the
following properties:
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(XS, a) is a complete preorder.
there are two acts such that f a g.
 A, f, x, y constant, x  a y  xAf  yAf
if f >a h and g >a h imply f g >a h
if x is constant, h >a x and h >a g imply h >a xg
then there exists a finite chain L, an L-valued necessity
measure on S and an L-valued utility function u, such that
a is representable by the pessimistic possibilistic criterion
UPES(f).
Merits and limitations
of qualitative decision theory
• Provides a foundation for possibility theory
• Possibility theory is justified by observing how a
decision-maker ranks acts
• Applies to one-shot decisions (no compensations/
accumulation effects in repeated decision steps)
• Presupposes that consecutive qualitative value
levels are distant from each other (negligibility
effects)
Quantitative possibility theory
• Membership functions of fuzzy sets
– Natural language descriptions pertaining to numerical
universes (fuzzy numbers)
– Results of fuzzy clustering
Semantics: metrics, proximity to prototypes
• Upper probability bound
– Random experiments with imprecise outcomes
– Consonant approximations of convex probability sets
Semantics: frequentist, subjectivist (gambles)...
Quantitative possibility theory
• Orders of magnitude of very small
probabilities
degrees of impossibility k(A) ranging on
integers
k(A) = n iff P(A) = en
• Likelihood functions (P(A| x), where x
varies) behave like possibility distributions
P(A| B) ≤ maxx  B P(A| x)
POSSIBILITY AS
UPPER PROBABILITY
• Given a numerical possibility distribution p, define
A}
P(p) = {Probabilities P | P(A) ≤ (A) for all
• Then, generally it holds that
(A) = sup {P(A) | P  P(p)}
N(A) = inf {P(A) | P  P(p)}
• So p is a faithful representation of a family of
probability measures.
From confidence sets to possibility
distributions
Consider a nested family of sets E1  E2 …  En
a set of positive numbers a1 …an in [0, 1]
and the family of probability functions
P = {P | P(Ei) ≥ ai for all i}.
P is always representable by means of a possibility measure.
Its possibility distribution is precisely
πx = mini max(µEi, 1 – ai)
Random set view
F
1
2
possibility levels
1 >  2 >  3 >… >  n
3
4
• Let mi = i – i+1
then m1 +… + mn = 1
A basic probability assignment (SHAFER)
• π(s) = ∑i: sAi mi (one point-coverage function)
• Only in the consonant case can m be recalculated from π
CONDITIONAL POSSIBILITY MEASURES
• A Coxian axiom
(A C) = (A |C)*(C),
with * = product
Then: (A |C) =(A C)/ (C)
N(A| C) = 1 – (Ac | C)
Dempster rule of conditioning (preserves s-maxitivity)
For the revision of possibility distributions: minimal change
of  when N(C) = 1.
It improves the state of information (reduction of
focal elements)
Bayesian possibilistic conditioning
(A |b C) = sup{P(A|C), P ≤ , P(C) > 0}
N(A |b C) = inf{P(A|C), P ≤ , P(C) > 0}
It is still a possibility measure
π(s |b C) = π(s)max(1, 1 /( π(s) + N(C)))
It can be shown that:
(A |b C) =(A C)/ ((A C) + N(Ac C))
N(A|b C) = N(A C) / (N(A C) + (Ac C))
= 1 – (Ac |b C)
For inference from generic knowledge based on observations
Possibility-Probability transformations
• Why ?
– fusion of heterogeneous data
– decision-making : betting according to a
possibility distribution leads to probability.
– Extraction of a representative value
– Simplified non-parametric imprecise
probabilistic models
Elementary forms of probability-possibility
transformations exist for a long time
• POSS PROB: Laplace indifference
principle
“ All that is equipossible is
equiprobable ”
= changing a uniform possibility
distribution into a uniform probability distribution
• PROB POSS: Confidence intervals
Replacing a probability distribution by an interval A with a
confidence level c.
– It defines a possibility distribution
– π(x) = 1 if x  A,
= 1 – c if x  A
Possibility-Probability transformations :
BASIC PRINCIPLES
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Possibility probability consistency: P ≤ 
Preserving the ordering of events :
P(A) ≥ P(B) (A) ≥ (B)
or elementary events only
p(x) > p(x') if and only if p(x) > p(x')
(order preservation)
Informational criteria:
from  to P: Preservation of symmetries
(Shapley value rather than maximal entropy)
from P to : optimize information content
(Maximization or minimisation of specificity
From OBJECTIVE probability to possibility :
• Rationale : given a probability p, try and preserve
as much information as possible
• Select a most specific element of the set
PI(P) = {:  ≥ P} of possibility measures
dominating P such that p (x) > p (x') iff p(x) >
p(x')
• may be weakened into :
p(x) > p(x') implies p (x) > p (x')
• The result is pi = j=i,…n pi
(case of no ties)
From probability to possibility : Continuous case
• The possibility distribution p obtained by transforming p
encodes then family of confidence intervals around the
mode of p.
• The -cut of p is the (1- )-confidence interval of p
• The optimal symmetric transform of the uniform
probability distribution is the triangular fuzzy number
• The symmetric triangular fuzzy number (STFN) is a
covering approximation of any probability with unimodal
symmetric density p with the same mode.
• In other words the -cut of a STFN contains the (1- )confidence interval of any such p.
From probability to possibility : Continuous case
• IL = {x, p(x) ≥ }
= [aL, aL+ L]
is the interval of length L
with maximal probability
• The most specific
possibility distribution
dominating p is π such
that L > 0, π(aL) = π(aL+
L) = 1 – P(IL).

p
L
aL
aL+ L
Possibilistic view of probabilistic
inequalities
• Chebyshev inequality defines a possibility
distribution that dominates any density with
given mean and variance.
• The symmetric triangular fuzzy number (STFN)
defines a possibility distribution that
optimally dominates any symmetric density
with given mode and bounded support.
From possibility to probability
• Idea (Kaufmann, Yager, Chanas):
–Pick a number  in [0, 1] at random
–Pick an element at random in the -cut of π.
a generalized Laplacean indifference principle : change
alpha-cuts into uniform probability distributions.
•Rationale : minimise arbitrariness by preserving the
symmetry properties of the representation.
•The resulting probability distribution is:
• The centre of gravity of the polyhedron P(p
•The pignistic transformation of belief functions (Smets)
•The Shapley value of the unanimity game N in game theory.
SUBJECTIVE POSSIBILITY
DISTRIBUTIONS
• Starting point : exploit the betting approach to
subjective probability
• A critique: The agent is forced to be additive by
the rules of exchangeable bets.
– For instance, the agent provides a uniform probability
distribution on a finite set whether (s)he knows nothing
about the concerned phenomenon, or if (s)he knows the
concerned phenomenon is purely random.
• Idea : It is assumed that a subjective probability
supplied by an agent is only a trace of the agent's
belief.
SUBJECTIVE POSSIBILITY
DISTRIBUTIONS
• Assumption 1: Beliefs can be modelled by belief
functions
– (masses m(A) summing to 1 assigned to subsets A).
• Assumption 2: The agent uses a probability
function induced by his or her beliefs, using the
pignistic transformation (Smets, 1990) or Shapley
value.
• Method : reconstruct the underlying belief
function from the probability provided by the
agent by choosing among the isopignistic ones.
SUBJECTIVE POSSIBILITY
DISTRIBUTIONS
– There are clearly several belief functions with a
prescribed Shapley value.
• Consider the least informative of those, in the
sense of a non-specificity index (expected
cardinality of the random set)
I(m) = ∑  m(A)card(A).
• RESULT : The least informative belief function
whose Shapley value is p is unique and consonant.
SUBJECTIVE POSSIBILITY
DISTRIBUTIONS
• The least specific belief function in the sense of
maximizing I(m) is characterized by

pi = j=1,n min(pj, pi).
• It is a probability-possibility transformation,
previously suggested in 1983: This is the unique
possibility distribution whose Shapley value is p.
• It gives results that are less specific than the
confidence interval approach to objective
probability.
Applications of quantitative possibility
• Representing incomplete probabilistic data for uncertainty
propagation in computations
• (but fuzzy interval analysis based on the extension
principle differs from conservative probabilistic risk
analysis)
• Systematizing some statistical methods (confidence
intervals, likelihood functions, probabilistic inequalities)
• Defuzzification based on Choquet integral (linear with
fuzzy number addition)
Applications of quantitative possibility
•
Uncertain reasoning : Possibilistic nets are a counterpart
to Bayesian nets that copes with incomplete data. Similar
algorithmic properties under Dempster conditioning
(Kruse team)
• Data fusion : well suited for merging heterogeneous
information on numerical data (linguistic, statistics,
confidence intervals) (Bloch)
• Risk analysis : uncertainty propagation using fuzzy
arithmetics, and random interval arithmetics when
statistical data is incomplete (Lodwick, Ferson)
• Non-parametric conservative modelling of imprecision in
measurements (Mauris)
Perspectives
Quantitative possibility is not as well understood as
probability theory.
• Objective vs. subjective possibility (a la De Finetti)
• How to use possibilistic conditioning in inference tasks ?
• Bridge the gap with statistics and the confidence interval
literature (Fisher, likelihood reasoning)
• Higher-order modes of fuzzy intervals (variance, …) and
links with fuzzy random variables
• Quantitative possibilistic expectations : decision-theoretic
characterisation ?
Conclusion
• Possibility theory is a simple and versatile tool for
modeling uncertainty
• A unifying framework for modeling and merging
linguistic knowledge and statistical data
• Useful to account for missing information in
reasoning tasks and risk analysis
• A bridge between logic-based AI and probabilistic
reasoning
Properties of inference |=p
•A |=π A if A ≠ Ø (restricted reflexivity)
•if A ≠ Ø, then A |=π Ø never holds (consistency preservation)
•The set {B: A |= π B} is deductively closed
-If A  B and C |=π A then C |=π B
(right weakening rule RW)
-If A |=π B and A |=π C then A |=π B C
(Right AND)
Properties of inference |=p
• If A |=π C ; B |=π C then A  B |=π C (Left OR)
• If A |=π B and A  B |=π C then A |=π C
(cut, weak transitivity )
(But if A normally implies B which normally implies C, then A
may not imply C)
• If A |=π B and if A |=π Cc is false, then A  C |=π B
(rational monotony RM)
If B is normally expected when A holds,then B is expected to hold when
both A and C hold, unless it is that A normally implies not C
REPRESENTATION THEOREM FOR POSSIBILISTIC
ENTAILMENT
•Let |= be a consequence relation on 2S x 2S
•Define an induced partial relation on subsets as
A > B iff A  B |= Bc for A ≠ 
•Theorem: If |= satisfies restricted reflexivity, right
weakening, rational monotony, Right AND and Left OR, then
A > B is the strict part of a possibility relation on events.
So a consequence relation satisfying the above properties
is representable by possibilistic inference, and induces a
complete plausibility preordering on the states.
A POSSIBILISTIC APPROACH
MODELING RULES
TO
• A generic rule « if A then B » is modelled by
(AB) > (Ac B).
• This is a constraint that delimits a set of possibility
distributions on the set of interpretations of the
language
• Applying the minimal specificity principle:
B).
(AB) = (ABc ) = (Ac Bc ) > (Ac
MODELLING A SET OF DEFAULT RULES as a
POSSIBILITY DISTRIBUTION
• ∆ = {Ai  Bi, i = 1,n}
• ∆ defines a set of constraints on possibility
distributions (Ai  Bi) > (Ai  ¬Bi), i = 1,…n
 • (∆) = set of feasible π's with respect to ∆
• One may compute * : the least specific
possibility distribution in (∆)
Plausible inference from a set of
default rules
What « ∆ implies A  B » means
• Cautious inference
∆ |= A  B iff
For all  (∆), (AB) > (Ac B).
• Possibilistic inference
∆ |=* A  B iff *(AB) > *(Ac B) for the least
specific possibility measure in (∆).
Leads to a stratification of ∆ according to N*(Ac B)
Possibilistic logic
• A possibilistic knowledge base is an ordered set of
propositional or 1st order formulas pi
• K = {(pi i), i = 1,n} where i > 0 is the level of
priority or validity of pi
i = 1 means certainty.
i = 0 means ignorance
• Captures the idea of uncertain knowledge in an
ordinal setting
Possibilistic logic
• Axiomatization:
All axioms of classical logic with weight 1
Weighted modus ponens
{(p ), (¬p  q )} |- (q min(,))
OLD! Goes back to Aristotle school
Idea: the validity of a chain of uncertain deductions is
the validity of its weakest link
Syntactic inference K |-(p ) is well-defined
Possibilistic logic
• Inconsistency becomes a graded notion
inc(K) = sup{, K |- (,)}
• Refutation and resolution methods extend
K |- (p ) iff K {(p 1)} |- (,)
• Inference with a partially inconsistent knowledge
base becomes non-trivial and nonmonotonic
K |-nt p iff K |- (p ) and  > inc(K)
Semantics of possibilistic logic
• A weighted formula has a fuzzy set of models .
• If A = [p] is the set of models of p (subset of S),
• |-(p ) means N(A) ≥ 
The least specific possibility distribution induced by |-(p )
is:
π(p )(s) = max(µA(s), 1 – )
= 1 if p is true in state s
= 1 –  if p is false in state s
Semantics of possibilistic logic
• The fuzzy set of models of K is the
intersection of the fuzzy sets of models of
{(pi i), i = 1,n}
• πK(s)= mini=1,n {1 – i | s  [pi]}
determined by the highest priority formula
violated by s
• The p. d. πK is the least informed state of
partial knowledge compatible with K
Soundness and completeness
• Monotonic semantic entailment follows Zadeh’s entailment
principle
K
|= (p, ) stands for πK ≤ π(p a)
Theorem: K |- (p, ) iff K |= (p )
• For the non-trivial inference under inconsistency:{(p 1)} 
K |-nt q iff (q  p) > (¬q  p)
Possibilistic vs. fuzzy logics
• Possibilistic logic
– Formulas are Boolean
– Truth is 2-valued
– Weighted formulas have fuzzy
sets of models
– Validity is many-valued
– degrees of validity are not
compositional except for
conjunctions
– Represents uncertainty
• Fuzzy logic (Pavelka)
– Formulas are non-Boolean
– Truth is many-valued
– Weighted formulas have crisp
sets of models (cuts)
– Validity is Boolean
– degrees of truth are
compositional
– represents real functions by
means of logical formulas
Example: IF BIRD THEN FLY; IF PENGUIN THEN BIRD;
IF PENGUIN THEN NOT-FLY
• K = {b  f, p  b, p  ¬f}
 = material implication
• K  {b} |- f;
K  {p} |-  contradiction
• using possibilistic logic:  < min(,)
K = {(b  f ), (p  b ), (p  ¬f )}
then K  {(b, 1)} |- (f ) and K  {(b, 1)} |-nt f
• Inc(K{(p, 1), (b, 1)} = 
• K  {(p, 1), (b, 1)} |- (¬f, min(,))
• Hence
K  {(p, 1), (b, 1)} |-nt ¬f