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First-order Logic Propositional logic (cont…)

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1 First-order Logic Propositional logic (cont…)
Syntax&Semantics of first-order logic Deducing hidden properties Describing actions

2 The knowledge base Percept sentences:
there is no smell in the square [1,1]  S1,1 there is no breeze in the square [1,1]  B1,1 there is no smell in the square [2,1]  S2,1 there is breeze in the square [2,1]  　B2,1 there is smell in the square [1,2]  S1,2 there is no breeze in the square [1,2]  B1,2

3 The knowledge base knowledge sentences:
If a square has no smell, then neither the square nor any of its adjacent squares can house a wumpus. R1: S1,1  W1,1  W1,2  W2,1 R2: S2,1  W1,1  W2,1  W2,2  W3,1 If there is smell in [1,2], then there must be a wumpus in [1,2] or in one or more of the neighboring squares. R3: S1,2  W1,3  W1,2  W2,2  W1,1 If a square has no breeze, then neither the square nor any of its adjacent squares can have a pit. R4: B1,1  P1,1  P1,2  P2,1 R5: B1,2  P1,1  P1,2 P2,2  P1,3 If there is breeze in [2,1], then there must be a pit in [2,1] or in one or more of the neighboring squares. R6: B2,1  P3,1  P2,1  P2,2  P1,1

4 Seven inference rules for propositional Logic
Modus Ponens And-Elimination And-Introduction Or-Introduction Double-Negation Elimination Unit Resolution Logic connectives:   ,  i 1  2 … n 1  2 … n 1, 2, …, n 1  2  …  n i      ,     ,       

5 Problem with propositional logic
Too many propositions  too many rules to define a competent agent The world is changing, propositions are changing with time.  do not know how many time-dependent propositions we will need have to go back and rewrite time-dependent version of each rule. The problem with proposition logic is that it only has one representational device: the proposition!!! The solutions to the problem is to introduce other logic first-order logic That can represent objects and relations between objects in addition to propositions.

6 First-order Logic What is first-order logic?
First-order logic contains: Objects: are things with individual identities and properties. e.g., people, houses, computers, numbers, Mike Jason, color, … Properties: are used to distinguish an object from other objects. e.g., tall, western style, multimedia, prime, English, red, … Relations: exist and hold among the objects. e.g., father of, bigger than, made after, equal, student of, … Functions: are relations in which there is only one “value” for a given “input”. e.g., brother of, increment of, forward, one more than, … Almost any fact can be thought of as referring to objects and properties or relations. For example: One plus two equals three. Objects: one, two, three, one plus one; Relations: equals; Function: plus. Squares neighboring the wumpus are smelly. Objects: squares, wumpus; Property: smelly; Relation: neighboring

7 Syntax of FOL: basic element
Constant symbols: refer to the same object in the same interpretation e.g. Mike Jason, 4, A, B, … Predicate symbols: refer to a particular relation in the model. e.g., Brother, >, Function symbols: refer to particular objects without using their names. Some relations are functional, that is, any given object is related to exactly one other object by the relation. (one-one relation) e.g., Cosine, FatherOf, Variables: substitute the name of an objec. e.g., x, y, a, b,… x, Cat(x)  Mammal(x) if x is a cat then x is a mammal. Logic connectives:  (not),  (and),  (or),  (implies), and  (equivalent) Quantifiers:  (universal quantification symbol),  (existential quantification symbol) x, for any x, … x, there is a x, … Equality: = e.g. Father(John) = Henry

8 Sentences: atomic vs complex
Atomic sentences = predicate(term1, term2, …termn) or term1 = term2 Term = function(term1, term2, …termn) or constant or variable E.g., Sister(Muxin, Yanbo) >(height(Muxin), height(Yanbo)) x, >(height(Yanbo), height(x)) Complex sentences: made from atomic sentences using connectives.  (not),  (and),  (or),  (implies), and  (equivalent) E.g., Sibling(Muxin, Yanbo)  Sibling(Yanbo, Muxin) >(Age(Muxin), Age(Yanbo))   >(Age(Muxin), Age(Yanbo) )

9 Truth in first-order logic
Sentences are true with respect to a model and an interpretation. Model contains objects and relations among them. E.g., a model: a family. objects in this model: Robert John, Rose John, Shelly John, Daivd John. relations: Husband(Robert, Rose), Wife(Rose, Robert), Father(Robert, David), …, Sister(Shelly, David), … Interpretation specifies referents for constant symbols -> objects predicate symbols -> relations function symbols -> functional relations An atomic sentence, predicate(term1, term2, …termn), is true iff the objects referred to by term1, term2, …termn are relation referred to by predicate. E.g, Father(Robert, David) is true iff Robert and David are two objects in the family model. Robert is David’s father (relation referred to by predicate Father)

10 Universal quantification
<variables> <sentence> x P is equivalent to the conjunction of instantiations of P E.g., Every student at CIS is smart: x At(x, CIS)  Smart(x) At(Suzuki, CIS)  Smart(x)  At(Abe, CIS)  Smart(x)  At(Honda, CIS)  Smart(x)  … Typically,  is the main connective with . Common mistake: using  as the main connective with . x At(x, CIS)  Smart(x) means Every student is at CIS and every student is smart.

11 Existential quantification
<variables> <sentence>  x P is equivalent to the disjunction of instantiations of P E.g., Someone at CIS is smart:  x At(x, CIS)  Smart(x) At(Suzuki, CIS)  Smart(x)  At(Abe, CIS)  Smart(x)  At(Honda, CIS)  Smart(x)  … Typically,  is the main connective with . Common mistake: using  as the main connective with .  x At(x, CIS)  Smart(x) is true if there is any student who is not at CIS. The uniqueness quantifier ! E.g., ! x King(x) means that there is only one King.

12 Property 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 E.g.,  x y Knows(x, y) “There is a person who knows everyone in the world” y  x Knows(x, y) “Everyone in the world is known by at least one person” Quantifier duality: each can be expressed using the other. . x Likes(x, iceCream)   x  Likes(x, iceCream) “Everyone likes ice cream” means that “There is no one who does not like ice cream” x Likes(x, carrot)  x  Likes(x, carrot) “Someone likes carrot” means that “Not everyone does not like carrot”

13 Knowledge base for the Wumpus world
Perception: Stench (variable s), Breeze (variable b), Glitter (variable g), Wall (variable u), Scream (variable v) b, g, u, v, t Percept([S, b, g, u, v], t)  Smelly(t) s, g, u, v, t Percept([s, B g, u, v], t)  Breeze(t) s, b, u, v, t Percept([s, b, G, u, v], t)  AtGoldRoom(t) Reflex: t AtGoldRoom(t)  Action(Grab, t) Reflex with internal state: do we have the gold already? t AtGoldRoom(t)  Holding(Gold, t)  Action(Grab, t)

14 Deducing hidden properties
Properties of locations: l, t At(Agent, l, t)  Smell(t)  Smell(l) l, t At(Agent, l, t)  Breeze(t)  Breeze(l) Diagnostic rule – infer cause from effect e. g. Squares are breezy near a pit y Breeze(y)   x Pit(x)  (x=y  Adjacent(x, y)) Causal rule – infer effect from cause x, y Pit(x)  (x=y  Adjacent(x, y))  Breeze(y)

15 Keeping track of world change
Diachronic rules: describe the way in which the world changes. Situation calculus: is one way to represent change in FOL. - Facts holds in situation rather than eternally. e.g. Holding(Gold, Now) rather than just Holding(Gold) Holding(Gold, Now) denotes a situation. where, Now is extra situation argument. - Situations are connected by the Result function Result(a, s). e.g. At(Agent, [1,1], S0)  At(Agent, [1,2], S1) Result(Forward, S0) = S1 p p w g p S0 S1 w g p A A p p

16 Describing actions Effect axioms – describe changes due to action
s AtGoldRoom(s)  Holding(Gold, Result(Grab, s)) x, s Holding(x, Result(Release, s)) Frame axioms – describe non-changes due to action a, x, s Holding(x, s)  (aRelease)  Holding(x, Result(a, s)) Successor-state axioms – combine the effect axioms and the frame axioms P true afterwards  [an action made P true  P true already and no action made P false] a, x, s Holding(x, Result(a, s))  [( (Present(x, s)  a=Grab )  (Holding(x, s)  aRelease) ]

17 Toward a goal Once the gold is found, the aim now is to return to the start as quickly as possible. Assuming that the agent now is holding “gold ”and has the goal of being at location [1,1].  s Holding(Gold, s)  GoalLocation([1,1], s) The presence of an explicit goal allows the agent to work out a sequence of actions that will achieve the goal.

18 Quiz. Complete the following truth table according to propositional syntax.
? S1 S2 S1 S1  S2 S1  S2 S1 S2 S1  S2 False True 　　　T T F

19 deduce 推論する hidden 隠れた property 特性 elimination 削除 introduction 導入 negation 否定 resolution 解決 dependent 従属関係の identity 正体 distinguish 識別する refer to 参照する predicate 述語 variable 変数 substitute 代わりに用いる quantify 〈…の〉量を定める equality 対等 existential 存在に関する complex 複雑 atomic 原子の referent 指示物 conjunction 連結 instantiation 具体例 disjunction 分離 reflex 反応（して行動） internal　 内部の causal 原因の diagnostic 診断の diachronic 通時的な calculus 微積分学 axiom 公理

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