Sentence Semantics II

Predicate Logic

Predicate Logic offers a way of analyzing the internal structure of propositions and representing sentence meaning.

Propositional Logic treats simple propositions as having no internal structure. Predicate Logic is used to help us think about the internal structure of simple propositions and how to represent sentence meaning.

Say we have a sentence such as p:

p = John loves Mary

Here is the grammar of a simple sentence. How do we represent the sentence meaning?

In Predicate Logic, this relationship is labelled like this:

Love (x, y)

x = John

y = Mary

Love (john, mary)

This gives us a simple, formal way of defining the proposition as a “predication.”

Predications consist of a Predicate (Capital Letter) and a set of arguments (small letter):

Predicate (arg1, …, argN)

One way of doing this is as follows:

P (x) means “x is a P”

For example, we could say “Japan is a country” like this:

Let’s say “Japan is a country.”

Country (japan)

This is a so-called one-place predicate.

A two-place predicate has two arguments.

For example, we could say that “John loves Mary” like this:

Loves (john, mary)

The Predicate is Love and the arguments are john and mary.

You could also have a three-place predicate. For example, we could say “John sends Mary his love” like this:

Send (john, mary, love)

Predications can also involve quantifiers. This allows us to think about the relations between sets of things. For example, think about these two sets — the set of crazy people and the set of linguists.

The set of  linguists: A

The set of crazy people: B

Perhaps we want to say that “All linguists are crazy.”

The word “all” indicates that one set is included in the other. In this case the set of linguists A is completely included in the set of crazy people B. There are no linguists who are not crazy. If you are a linguist, you are crazy.

Every member of the set of linguists is included in the set of crazy people.

This is how it is represented in Predicate Logic:

∀x (Linguist (x) → Crazy (x))

The upside-down A means “for all” and it allows us to look at everything that is x and check them off against the sets of linguists and crazy people. In this formula, we are saying that for every individual x that is in the set of linguists, that individual is also in the set of crazy people. Of course, we hope this proposition is false!

Here is another example. Perhaps we want to say “All men love Mary.” We could do it like this:

∀x (Man (x) → Love (x, mary))

Now maybe only some men love Mary and only some linguists are crazy. How do we represent that in Predicate Logic?

Some linguists are crazy = ∃x (Linguist (x) & Crazy (x))

The backward E indicates that two sets intersect.

Two sets intersect:  A = the set of linguists; B = the set of crazy people. At least one member of the set of linguists is in the set of crazy people. Or A = the set of men; B = the set of individuals who love Mary. At least one member of the set of men is in the set of individuals who love Mary. Or A = the set of girls; B = the set of individuals who live in Tsuru. At least one member of the set of girls is in the set of individuals who live in Tsuru.

For the set of individuals x, there will be at least one individual who is both in the set of linguists and in the set of crazy people.

Or perhaps we want to say “A girl lives in Tsuru.” We could do it like this:

∃x (Girl (x) & Lives_in (x, tsuru))

Or perhaps we want to say “No students like Predicate Logic.” We could do it like this:

¬∃x (Student (x) & Like (x, predicate_logic))

In this case the sets do not intersect at all. It is not the case that there is at least one individual who is both in the set of students and in the set of individuals who like Predicate Logic.

Predicate Logic allows us to analyze the internal structure of propositions and give a detailed formal representation of sentence meaning.