Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic:...
Transcript of Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic:...
![Page 1: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/1.jpg)
Modal logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 2
![Page 2: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/2.jpg)
What is Modal Logic?
Narrowly, traditionally: modal logic studies reasoning that involvesthe use of the expressions necessarily and possibly.More widely: modal logic covers a family of logics with similar rulesand a variety of different symbols.
Logic Symbols Expressions SymbolizedModal Logic ✷ It is necessary that ...
⋄ It is possible that ...Deontic Logic O It is obligatory that ...
P It is permitted that ...F It is forbidden that ...
Temporal Logic G It will always be the case that ...F It will be the case that ...H It has always been the case that ...P It was the case that ...
Doxastic Logic Bx x believes that ...
Benzmuller/Rojas, 2014—–Artificial Intelligence 3
![Page 3: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/3.jpg)
History
! Aristotles (356-323 BCE): developed a modal syllogistic inbook I of his Prior Analytics, which Theophrastus attemptedto improve
! Avicenna (980-1037): developed earliest formal system ofmodal logic
! William of Ockham (1287-1347) and John Duns Scotus(1266-1308): informal modal reasoning (about essence)
! C.I. Lewis (1883-1964): founded modern modal logic
! Ruth C. Barcan (1921-2012): first axiomatic systems ofquantified modal logic
! Saul Kripke: Kripke semantics for modal logics; possibleworldsßemantics
! A.N. Prior: created modern temporal logic in 1957
! Vaughan Pratt: introduced dynamic logic in 1976.
Benzmuller/Rojas, 2014—–Artificial Intelligence 4
![Page 4: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/4.jpg)
Further Reading
Garson, James, Modal Logic, The Stanford Encyclopedia of Philosophy(Spring 2013 Edition), Edward N. Zalta (ed.),<http://plato.stanford.edu/archives/spr2013/entries/logic-modal/>.
Benzmuller/Rojas, 2014—–Artificial Intelligence 5
![Page 5: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/5.jpg)
Modal Logic: Applications
Modal logics have been used in artificial intelligence applications tomodel
! Knowledge (including common knowledge)
! Belief (including common knowledge)
! Actions, goals, and intentions
! Ability and Obligations
! Time
! . . .
There are many further applications, also in other disciplines,including philosophy, linguistics, mathematics, computer science,. . . arts, poetry
(Here are some nice slides for further reading; see also the slides ofAndreas Herzig)
Benzmuller/Rojas, 2014—–Artificial Intelligence 6
![Page 6: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/6.jpg)
Modal Logic: Motivation
Material implication seems actually quite unintuitive:
ϕ⇒ ψ iff ¬ϕ ∨ ψ
Problem with material implication in many applications: see e.g.Dorothy Edgington’s Proof of the Existence of God:
! If God does not exist, then it’s not the case that if I pray, myprayers will be answered.
¬G ⇒ ¬(P ⇒ A)
! I do not pray: ¬P
! It follows: God exists. G
Benzmuller/Rojas, 2014—–Artificial Intelligence 7
![Page 7: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/7.jpg)
Modal Logic: Motivation
! If God does not exist, then it’s not the case that if I pray, myprayers will be answered.
¬g ⇒ ¬(p ⇒ a)
! I do not pray: ¬p
! It follows: God exists. g
In TPTP syntax:
fof(ax1,axiom,((~ g) => ~ (p => a))).
fof(ax2,axiom,(~ p)).
fof(c,conjecture,(g)).
Benzmuller/Rojas, 2014—–Artificial Intelligence 8
![Page 8: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/8.jpg)
Modal Logic: Motivation
Benzmuller/Rojas, 2014—–Artificial Intelligence 9
![Page 9: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/9.jpg)
Modal Logic: Motivation
Lewis instead proposed the use of strict implication:
ϕ⇒ψ iff ¬✸(ϕ ∧ ¬ψ)
ϕ implies ψ iff it is not possible that ϕ and ¬ψ are true.
Benzmuller/Rojas, 2014—–Artificial Intelligence 10
![Page 10: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/10.jpg)
Modal Logic
Modal Logic: Syntax
! any basic propositional symbol p ∈ P is a modal logic formula
! if ϕ and ψ are modal logic formulas, then so are ¬ϕ, ϕ ∨ ψ,ϕ ∧ ψ, and ϕ⇒ ψ
! if ϕ is a modal logic formula, then so are ✷ϕ and ✸ϕ
Prominent modal logics are constructed from a weak logic called K(after Saul Kripke).
Theorems of Basic Modal Logic K
! if ϕ is a theorem of propositional logic, then ϕ is also atheorem of K
! Necessitation Rule: If ϕ is a theorem of K, then so is ✷ϕ
! Distribution Axiom: ✷(ϕ⇒ ψ) ⇒ (✷ϕ⇒ ✷ψ)
Benzmuller/Rojas, 2014—–Artificial Intelligence 11
![Page 11: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/11.jpg)
Modal Logics beyond K
From base logic K we can derive at other modal logics by addingfurther axioms
Name Axioms
K ✷(ϕ⇒ ψ) ⇒ (✷ϕ⇒ ✷ψ)
M(orT ) ✷ϕ⇒ ϕ
D ✷ϕ⇒ ✸ϕ
B ϕ⇒ ✷✸ϕ
4 ✷ϕ⇒ ✷✷ϕ
5 ✸ϕ⇒ ✷✸ϕ
A variety of logics may be developed using K as a foundation byadding combinations of the above axioms.
Benzmuller/Rojas, 2014—–Artificial Intelligence 12
![Page 12: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/12.jpg)
Modal Logics beyond K
Many philosophers consider logic S5 (K+M+4+5) an adequatechoice for necessity.
In S5, **. . .✷ = ✷ and **. . .✸ = ✸, where each * is either ✷ or✸. This amounts to the idea that strings containing both boxesand diamonds are equivalent to the last operator in the sequence.Saying that it is possible that A is necessary is the same as sayingthat A is necessary.
Modal logic can be extended to multi-modal logic, where the ✷
and ✸ operators are annotated with the identifier of the agent whohas that knowledge; see wise men puzzle above.
Benzmuller/Rojas, 2014—–Artificial Intelligence 13
![Page 13: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/13.jpg)
Modal Logic Cube
K KB
M B = MB
S4 S5 = M5 ≡ MB5 ≡ M4B5
≡ M45 ≡ M4B ≡ D4B
≡ D4B5 ≡ DB5
K4 KB5 ≡ K4B5 ≡ K4B
D
D4
K45
D45
DB
K5
D5
K
M4
5B
modal cube reproduced from J. Garson, Modal Logic, SEP 2009
Benzmuller/Rojas, 2014—–Artificial Intelligence 14
![Page 14: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/14.jpg)
Can you represent and solve the following problem?
Wise Men Puzzle
Once upon a time, a kingwanted to find the wisestout of his three wisest men.He arranged them in a circleand told them that he wouldput a white or a blackspot on their foreheads andthat one of the three spotswould certainly be white.The three wise men couldsee and hear each other but,of course, they could notsee their faces reflected any-where. The king, then, as-ked to each of them to findout the color of his ownspot. After a while, the wi-sest correctly answered thathis spot was white.
How could he know that?
Benzmuller/Rojas, 2014—–Artificial Intelligence 15
![Page 15: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/15.jpg)
Can you represent and solve the following problem?
Wise Men Puzzle
Once upon a time, a kingwanted to find the wisestout of his three wisest men.He arranged them in a circleand told them that he wouldput a white or a blackspot on their foreheads andthat one of the three spotswould certainly be white.The three wise men couldsee and hear each other but,of course, they could notsee their faces reflected any-where. The king, then, as-ked to each of them to findout the color of his ownspot. After a while, the wi-sest correctly answered thathis spot was white.
How could he know that?
Benzmuller/Rojas, 2014—–Artificial Intelligence 15
![Page 16: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/16.jpg)
Can you represent and solve the following problem?
Wise Men Puzzle
Once upon a time, a kingwanted to find the wisestout of his three wisest men.He arranged them in a circleand told them that he wouldput a white or a blackspot on their foreheads andthat one of the three spotswould certainly be white.The three wise men couldsee and hear each other but,of course, they could notsee their faces reflected any-where. The king, then, as-ked to each of them to findout the color of his ownspot. After a while, the wi-sest correctly answered thathis spot was white.
How could he know that?
✷fool ws a ∨ ws b ∨ ws c
✷fool ϕ⇒ ✷a ϕ
✷fool ϕ⇒ ✷b ϕ
. . .
¬✷a ws a
¬✷b ws b
Query: ✷c ws c
Benzmuller/Rojas, 2014—–Artificial Intelligence 15
![Page 17: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/17.jpg)
Kripke Style Semantics
Benzmuller/Rojas, 2014—–Artificial Intelligence 16
![Page 18: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/18.jpg)
Kripke Style Semantics
A Modal Frame F = ⟨W ,R , v⟩
. . . consists of set of possible worlds W , a binary accessibilityrelation R between worlds, and an evaluation function v forassigning truth values to the basic propositional symbols(v : PropSym ×W −→ {T ,F}).
Truth of a modal formula ϕ for a frame F and a world w
F ,w |= p iff v(p,w)
F ,w |= ¬ϕ iff F ,w |= ϕ
F ,w |= ϕ ∨ ψ iff F ,w |= ϕ or F ,w |= ψ
F ,w |= ϕ ∧ ψ iff F ,w |= ϕ and F ,w |= ψ
F ,w |= ϕ⇒ ψ iff F ,w |= ϕ or F ,w |= ψ
F ,w |= ✷ϕ iff F ,w ′ |= ϕ for all w ′ with wRw ′
F ,w |= ✸ϕ iff there exists w ′ with wRw ′ s.t. F ,w ′ |= ϕ
Benzmuller/Rojas, 2014—–Artificial Intelligence 17
![Page 19: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/19.jpg)
Kripke Style Semantics
Benzmuller/Rojas, 2014—–Artificial Intelligence 18
![Page 20: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/20.jpg)
Kripke Style Semantics
Truth of a modal formula (in base modal logic K)
A modal formula ϕ is true (or valid) iff it is true for all frames Fand all worlds w .
Exercises:
! Show that the Distribution axiom ✷(ϕ⇒ ψ) ⇒ (✷ϕ⇒ ✷ψ)is valid in logic K.
! Show that axiom T ✷ϕ⇒ ϕ is valid iff the accessibilityrelation R is reflexive.
Benzmuller/Rojas, 2014—–Artificial Intelligence 19
![Page 21: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/21.jpg)
Kripke Style Semantics
We have the following correspondences
Name Axioms Condition on R
K ✷(ϕ⇒ ψ) ⇒ (✷ϕ⇒ ✷ψ) none
M(orT ) ✷ϕ⇒ ϕ reflexive
D ✷ϕ⇒ ✸ϕ serial
B ϕ⇒ ✷✸ϕ symmetric
4 ✷ϕ⇒ ✷✷ϕ transitive
5 ✸ϕ⇒ ✷✸ϕ euclidean
Benzmuller/Rojas, 2014—–Artificial Intelligence 20
![Page 22: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/22.jpg)
Theorem Proving in Propositional Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 21
![Page 23: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/23.jpg)
Theorem Proving in Propositional Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 22
![Page 24: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/24.jpg)
Theorem Proving in Propositional Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 23
![Page 25: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/25.jpg)
Theorem Proving in Propositional Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 24
![Page 26: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/26.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 25
![Page 27: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/27.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 26
![Page 28: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/28.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 27
![Page 29: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/29.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 28
![Page 30: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/30.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 29
![Page 31: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/31.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 30
![Page 32: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/32.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 31
![Page 33: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/33.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 32
![Page 34: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/34.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 33
![Page 35: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/35.jpg)
Theorem Proving in Propositional Modal Logic
Benzmuller/Rojas, 2014—–Artificial Intelligence 34
![Page 36: Modal logic - UserPagespage.mi.fu-berlin.de/.../slides/lecture-6-A-Modallogic.pdfModal Logic: Applications Modal logics have been used in artificial intelligence applications to model!](https://reader033.fdocuments.net/reader033/viewer/2022052408/5f0f5ee27e708231d443d338/html5/thumbnails/36.jpg)
Exercise
Once upon a time, a king wanted to find the wisest out of his twowisest men. He told them that he would put a white or a blackspot on their foreheads and that one of the two spots wouldcertainly be white. The two wise men could see and hear eachother but, of course, they could not see their faces reflectedanywhere. The king, then, asked to each of them to find out thecolor of his own spot. After a while, the wisest correctly answeredthat his spot was white.
Exercise: Encode this situation in propositional modal logic.
Benzmuller/Rojas, 2014—–Artificial Intelligence 35