
Introduction to Logic

Tools for Thought

In Propositional Logic, sentences are constructed from a basic vocabulary of propositional constants. In Relational Logic, there are no propositional constants; instead we have object constants, relation constants, and variables.
In our examples here, we write both variables and constants as strings of letters, digits, and a few nonalphanumeric characters (e.g. "_"). By convention, variables begin with letters from the end of the alphabet (viz. u, v, w, x, y, z). Examples include x, ya, and z_2. By convention, all constants begin with either alphabetic letters (other than u, v, w, x, y, z) or digits. Examples include a, b, 123, comp225, and barack_obama.
Note that there is no distinction in spelling between object constants and relation constants. The type of each such word is determined by its usage or, in some cases, in an explicit specification.

As we shall see, relation constants are used in forming complex expressions by combining them with an appropriate number of arguments. Accordingly, each relation constant has an associated arity, i.e. the number of arguments with which that relation constant can be combined. A relation constant that can combined with a single argument is said to be unary; one that can be combined with two arguments is said to be binary; one that can be combined with three arguments is said to be ternary; more generally, a relation constant that can be combined with n arguments is said to be nary.

A vocabulary consists of a set of object constants, a set of relation constants, and an assignment of arities for each of the relation constants in the vocabulary. (Note that this definition here is slightly nontraditional. In many textbooks, a vocabulary (sometimes called a signature) includes a specification of relation constants but not object constants, whereas our definition here includes both types of constants.)

A term is defined to be a variable or an object constant. Terms typically denote objects presumed or hypothesized to exist in the world; and, as such, they are analogous to noun phrases in natural language, e.g. Joe or someone.
There are three types of sentences in Relational Logic, viz. relational sentences (the analog of propositions in Propositional Logic), logical sentences (analogous to the logical sentences in Propositional Logic), and quantified sentences (which have no analog in Propositional Logic).

A relational sentence is an expression formed from an nary relation constant and n terms. For example, if q is a relation constant with arity 2 and if a and y are terms, then the expression shown below is a syntactically legal relational sentence. Relational sentences are sometimes called atoms to distinguish them from logical and quantified sentences.
q(a, y)

Logical sentences are defined as in Propositional Logic. There are negations, conjunctions, disjunctions, implications, and equivalences. See below for examples.
Negation:   (¬p(a)) 
Conjunction:   (p(a) ∧ q(b, c)) 
Disjunction:   (p(a) ∨ q(b, c)) 
Implication:   (p(a) ⇒ q(b, c)) 
Biconditional:   (p(a) ⇔ q(b, c)) 
Note that the syntax here is exactly the same as in Propositional Logic except that the elementary components are relational sentences rather than proposition constants.

Quantified sentences are formed from a quantifier, a variable, and an embedded sentence. The embedded sentence is called the scope of the quantifier. There are two types of quantified sentences in Relational Logic, viz. universally quantified sentences and existentially quantified sentences.
A universally quantified sentence is used to assert that all objects have a certain property. For example, the following expression is a universally quantified sentence asserting that, if p holds of an object, then q holds of that object and itself.
(∀x.(p(x) ⇒ q(x,x)))
An existentially quantified sentence is used to assert that some object has a certain property. For example, the following expression is an existentially quantified sentence asserting that there is an object that satisfies p and, when paired with itself, satisfies q as well.
(∃x.(p(x) ∧ q(x,x)))

Note that quantified sentences can be nested within other sentences. For example, in the first sentence below, we have quantified sentences inside of a disjunction. In the second sentence, we have a quantified sentence nested inside of another quantified sentence.
((∀x.p(x)) ∨ (∃x.q(x,x)))
(∀x.(∃y.q(x,y)))

As with Propositional Logic, we can drop unneeded parentheses in Relational Logic, relying on precedence to disambiguate the structure of unparenthesized sentences. In Relational Logic, the precedence relations of the logical operators are the same as in Propositional Logic, and quantifiers have higher precedence than logical operators.

The following examples show how to parenthesize sentences with both quantifiers and logical operators. The sentences on the right are partially parenthesized versions of the sentences on the left. (To be fully parenthesized, we would need to add parentheses around each of the sentences as a whole.)
∀x.p(x) ⇒ q(x) 

(∀x.p(x)) ⇒ q(x) 
∃x.p(x) ∧ q(x) 

(∃x.p(x)) ∧ q(x) 
Notice that, in each of these examples, the quantifier does not apply to the second relational sentence, even though, in each case, that sentence contains an occurrence of the variable being quantified. If we want to apply the quantifier to a logical sentence, we must enclose that sentence in parentheses, as in the following examples.
∀x.(p(x) ⇒ q(x)) 
∃x.(p(x) ∧ q(x)) 

An expression in Relational Logic is ground if and only if it contains no variables. For example, the sentence p(a) is ground, whereas the sentence ∀x.p(x) is not.
An occurrence of a variable is free if and only if it is not in the scope of a quantifier of that variable. Otherwise, it is bound. For example, y is free and x is bound in the following sentence.
∃x.q(x,y)
A sentence is open if and only if it has free variables. Otherwise, it is closed. For example, the first sentence below is open and the second is closed.
p(y) ⇒ ∃x.q(x,y)
∀y.(p(y) ⇒ ∃x.q(x,y))

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