### 12.3.2 Interpreting Variables

When a variable appears in a clause, the clause is *true* in an
interpretation only if the clause is *true* for all possible values of
that variable. The variable is said to be **universally
quantified** within the scope of the clause. If a
variable *X* appears in a clause *C*, then claiming that *C* is
*true* in an interpretation means that *C* is *true* no matter which
individual from the domain is denoted by *X*.

To formally define semantics of variables, a **variable
assignment**, *ρ*, is a function from
the set of variables into the domain *D*. Thus, a variable assignment
assigns an element of the domain to each variable. Given *φ* and
a variable assignment *ρ*, each term denotes an individual in the
domain. If the term is a constant, the individual denoted is given by
*φ*. If the term is a variable, the individual denoted is given by
*ρ*. Given an interpretation and a variable assignment, each atom
is either true or false, using the same definition as earlier. Similarly,
given an interpretation and a variable assignment, each clause is
either true or false.

A clause is true in an interpretation if it is true for all variable
assignments. This is called a **universal
quantification**. The variables are
said to be **universally quantified** in the scope of the clause. Thus,
a clause is false in an interpretation means there is a variable
assignment under which the clause is false. The scope of the
variable is the whole clause, which means that the same variable
assignment is used for all instances of a variable in a clause.

**Example 12.6:**The clause

part_of(X,Y) ←in(X,Y).

is false in the interpretation of Example 12.4, because under the
variable assignment with
*X* denoting Kim and *Y* denoting Room 123, the clause's body
is true and the clause's head
is false.

The clause

in(X,Y) ←part_of(Z,Y) ∧in(X,Z).

is true, because in all variable assignments where the body is true, the head is also true.

Logical consequence is defined as in
Section 5.1: ground body *g* is a
**logical consequence** of *KB*,
written *KB g*, if *g* is *true* in every model of *KB*.

**Example 12.7:**Suppose the knowledge base

*KB*is

*in(kim,r123).*

*part_of(r123,cs_building).*

*in(X,Y) ←*

*part_of(Z,Y) ∧*

*in(X,Z).*

The interpretation defined in Example 12.4 is a model of *KB*,
because each clause is true in that interpretation.

*KB in(kim,r123)*, because this is stated explicitly in the
knowledge base. If
every clause of *KB* is true in an interpretation, then *in(kim,r123)*
must be true in that interpretation.

*KB in(kim,r023)*. The interpretation defined in
Example 12.4 is a model of *KB*, in which *in(kim,r023)* is
false.

*KB part_of(r023,cs_building)*. Although
*part_of(r023,cs_building)* is true in the interpretation of
Example 12.4, there is another model of *KB* in which
*part_of(r023,cs_building)* is false. In particular, the
interpretation which is like the interpretation of
Example 12.4, but where*π(part_of)(⟨φ(r023),φ(cs_building)⟩)=*false,

is a model of *KB* in which *part_of(r023,cs_building)* is false.

*KB in(kim,cs_building)*. If the clauses in *KB* are true in
interpretation *I*, it must be the case that *in(kim,cs_building)*
is true in *I*, otherwise there is an instance of the third clause of
*KB* that is false in *I* - a contradiction to *I* being a model of *KB*.

Notice how the semantics treats variables appearing in a clause's body but not in its head (see Example 12.8).

**Example 12.8:**In Example 12.7, the variable

*Y*in the clause defining

*in*is universally quantified at the level of the clause; thus, the clause is true for all variable assignments. Consider particular values

*c*for

_{1}*X*and

*c*for

_{2}*Y*. The clause

*in(c*

_{1},c_{2}) ←*part_of(Z,c*

_{2}) ∧*in(c*

_{1},Z).is *true* for all variable assignments to *Z*. If there exists a variable
assignment *c _{3}* for

*Z*such that

*part_of(Z,c*is

_{2}) ∧in(c_{1},Z)*true*in an interpretation, then

*in(c*must be

_{1},c_{2})*true*in that interpretation. Therefore, you can read the last clause of Example 12.7 as "for all

*X*and for all

*Y*,

*in(X,Y)*is

*true*if there exists a

*Z*such that

*part_of(Z,Y) ∧in(X,Z)*is

*true*."

When we want to make the quantification explicit, we write *∀X
p(X)*, which reads, "**for all** *X*, *p(X)*," to mean *p(X)* is true for
every variable assignment for *X*. We write *exist X
p(X)* and read "**there exists** *X* such that *p(X)*" to mean *p(X)* is true for
some variable assignment for *X*. *X* is said to be an **existentially
quantified variable**.

The rule *P(X) ←Q(X,Y)* means

∀X ∀Y (P(X) ←Q(X,Y)),

which is equivalent to

∀X (P(X) ←exist Y Q(X,Y)).

Thus, free variables that only appear in the body are existentially quantified in the scope of the body.

It may seem as though there is something peculiar about talking about
the clause being *true* for cases where it does not make sense.

**Example 12.9:**Consider the clause

*in(cs422,love) ←*

*part_of(cs422,sky) ∧*

*in(sky,love).*

where *cs422* denotes a course, *love* denotes an abstract
concept, and *sky* denotes the sky. Here, the clause
is vacuously *true* in the intended interpretation according to the truth table for *←*,
because the clause's right-hand side is *false* in the intended interpretation.

As long as whenever the head is non-sensical, the body is also,
the rule can never be used to prove anything non-sensical. When
checking for the truth of a clause, you must only be concerned with
those cases in which the clause's body is *true*.
Using the convention that a clause is true whenever the body is false,
even if it does not make sense, makes the semantics simpler and
does not cause any problems.