Semantics 1, WiSe 2016/17, Week 4

Computing the truth value of atomic formulae

The following video presents the step-by-step computation of the truth value of two atomic formulae. The example uses a model based on Shakespeare's play Macbeth. The two formulae are:

• kill(macbeth,duncan)
• kill(lady-macbeth,macbet)

Formulae with connectives

The following video presents the step-by-step computation of the truth value of two formulae with connectives. The example uses a model based on Shakespeare's play Macbeth. The two formulae are:

• ¬ king(lady-macbeth)
• king(duncan) ∨ king(lady-macbeth)

The next video shows how the truth value of a more complex formula can be computed. The example contains two connectives:

kill(malcom,lady-macbeth) ∨ ¬thane(macbeth)

The video shows two different methods: top down and bottom up.

Truth tables

The following material is an adapted form of material created by student participants of the project e-Learning Resources for Semantics (e-LRS). Involved participants: Lisa, Marthe, Elisabeth, Isabelle.

Truth tables for connectives

AND (∧)

Symbol: ∧
Sentence: Harry is a student and Snape is a teacher.
Formulae: student(harry) ∧ teacher(snape)

A conjunction p ∧ q it true if and only if p is true and q is true.

Truthtable AND

OR (∨)

Symbol: ∨
Sentence: Harry is a student or Snape is a teacher.
Formulae: student(harry) ∨ teacher(snape)

A disjunction p ∨ q is true if and only if p is true or q is true (or both).

Truthtable OR

IF/THEN (⊃, →)

Symbol: ⊃, → (Note: We use the symbol ⊃ in the textbook as it is more common in the logical literature.)
Sentence: If Harry is a student then Snape is a teacher.
Formula: student(harry) ⊃ teacher(snape)

An implication p ⊃ q is true if and only if p is false or q is true (or both).
In other words: An implication p ⊃ q is true if and only if whenever p is true, q is true as well.

Truthtable IF/THEN

NOT (¬)

Symbol: ¬
Sentence: Harry is not a student.
Formula: ¬student(harry)

A negated formula ¬p is true if and only if p is false.

Example: Only if student(harry) is false, ¬student(harry) is true.

Truthtable NOT

Truth tables for complex formulae

Truth tables are also useful to compute the truth value of complex formulae. This is shown in the following podcast, created by Lisa Günthner.

Interpretation of atomic formulae

Interpret the following formulae as true or false. If you have not defined these relations or properties in your model use the ones given in a previous exercise.

• father-of-someone(paul,lisa)

Check your answers

[[father-of-someone(paul,lisa)]] = true iff
< [[paul]], [[lisa]] > ∈ [[father-of-someone]] iff
< I(paul), I(lisa) > ∈ I(father-of-someone) iff
< Paul, Lisa> ∈ {<Paul, Tom>,<Paul, Lisa>}.

Since this is the case, the formula is true.

• blonde(walter)

Check your answers

[[blonde(walter)]] = true iff
< I(walter) > ∈ I(blonde) iff
< Walter > ∈ {< Alice >,< Lisa >}.

Since this is not the case, the overall formula is false.

• enjoy-watching-football-together(alice,tom)

Check your answers

[[enjoy-watching-football-togehter(alice,tom)]] = true iff
< I(alice), I(tom) > ∈ I(enjoy-watching-football-together) iff
< Alice, Tom > ∈ {<Alice, Paul>,<Paul, Alice>,<Alice, Lisa>,<Lisa, Alice>,<Alice, Tom>,<Tom, Alice>,<Paul, Lisa>,<Lisa, Paul>,<Paul, Tom>,<Tom, Paul>,<Tom, Lisa>,<Lisa, Tom>}

Since this is the case, the formula is true.

Interpretation of formulae with logical connectives

Consider these two natural language sentences. While keeping in mind the scenario given in a previous exercise, create complex formulae with logical connectives and compute the interpretation, respectively.

a.) Alice is a dog and Lisa and Tom enjoy watching football together.

Check your answers

Sentence: Alice is a dog and Lisa and Tom enjoy watching football together.

Here the interpretation in predicate logic notation:

[[dog (Alice) Ʌ enjoy-watching-football-together (Lisa,Tom)]] = false

because [[dog (Alice)]]= false

because I(Alice)= <Alice> and <Alice> is NOT an element of I(dog)

and [[enjoy-watching-soccer-together (Lisa,Tom)]] = true

because I(Lisa)= <Lisa>, I(Tom)= <Tom> and <Lisa,Tom> is in the set of I(enjoy-watching-football-together).

Conjunction (Ʌ): Both atomic formulae have to be true in order for the complex formula to be true.

b.) Tom is not Paul's daughter or Tom is tall.

Check your answers

Sentence: Tom is not Paul's daughter or Tom is tall.

Here the interpretation in predicate logic notation:

[[¬daughter-of-someone (Tom,Paul) v tall(Tom)]] = true

because [[¬daughter-of-someone (Tom,Paul)]]= true

because I(Tom)= <Tom>, I(Paul)= <Paul> and <Tom,Paul> is NOT in the set of I(daughter-of-someone)

and [[tall(Tom)]] = false

because I(Tom)= <Tom> and <Tom> is NOT an element of I(tall).

Disjunction (v): At least one of the atomic formulae has to be true in order for the complex formula to be true.

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• the page of Semantics 1 (WiSe 16/17)
• the exercises for chapter 2
• the material for chapter 2
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