# Formal Logic/Predicate Logic/Formal Syntax

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Free and Bound Variables → |

# Formal Syntax[edit | edit source]

In The Predicate Language, we informally described our sentential language. Here we give its **formal syntax** or grammar. We will call our language . This is an expansion of the sentential language and will include as a subset.

## Vocabulary[edit | edit source]

*Variables:*Lower case letters 'n'–'z' with a natural number subscript. Thus the variables are:

*Operation letters:*Lower case letters 'a'–'m' with (1) a natural number superscript and (2) a natural number subscript.

- A
*constant symbol*is a zero-place operation letter. This piece of terminology is not completely standard.

*Predicate letters:*Upper case letters 'A'–'Z' with (1) a natural number superscript and (2) a natural number subscript.

- A
*sentence letter*is a zero-place predicate letter.

*Sentential connectives*:

*Quantifiers*:

- Grouping symbols:

The superscripts on operation letters and predicate letters indicate the number of places and are important for formation rules. The subscripts on variables, operation letters, and predicate letters are to ensure an infinite supply of symbols in these classes. On a subsequent page we will abbreviate away most superscript use by letting the context make the number of places clear. We will also abbreviate away most subscript use by letting a symbol without a subscript abbreviate one with the subscript '0'. For now, though, we stick with the unabbreviated form.

The sentence letters of sentential logic are zero-place predicate letters, namely, predicate letters with the superscript '0'. The vocabulary of , the sentential logic formal language, includes zero-place predicate letters, sentential connectives, and grouping symbols.

## Expressions[edit | edit source]

Any string of symbols from is an *expression*. Not all expressions are grammatically well-formed. The primary well-formed expression is a formula. However, there are also well-formed entities that are smaller than formulae, namely quantifier phrases and terms.

## Formation rules[edit | edit source]

### Quantifier phrases[edit | edit source]

A *quantifier phrase* is a quantifier followed by a variable. The following are examples:

### Terms[edit | edit source]

An expression of is a *term* of if and only if it is constructed according to the following rules.

- A variable is a term.

- A constant symbol (zero-place operation letter, i.e., an operation letter with the superscript '0') is a term.

- If is an
*n*-place operation letter (*n*greater than 0) and are terms, then

- is a term.

A *name* is a term with no variables.

### Formulae[edit | edit source]

An expression of is a *well-formed formula* of if and only if it is constructed according to the following rules.

- A sentence letter (a zero-place predicate letter) is a well-formed formula.

- If is an
*n*-place predicate letter (*n*greater than 0) and are terms, then

- is a well-formed formula.

- If and are well-formed formulae, then so are each of:

- If is a well-formed formula and is a variable, then each of the following is a well-formed formula:

In general, we will use 'formula' as shorthand for 'well-formed formula'. We will see in the section Free and Bound Variables that only some formulae are sentences.

## Additional terminology[edit | edit source]

A few of these terms are repeated from above. All definitions from the sentential logic additional terminology section apply here except the definitions of 'atomic formula' and 'molecular formula'. These latter two terms are redefined below.

A *constant symbol* is a zero-place operation letter. (Note that different authors will vary on this.)

A *name* is a term in which no variables occur. (Note that different authors will vary on this. Some use 'name' only for zero-place operation letters, and some prefer to avoid the word altogether.)

A *sentence letter* is a zero-place predicate letter.

The *universal quantifier* is the symbol . The *existential quantifier* is the symbol .

A *quantified formula* is a formula that begins with a left parenthesis followed by a quantifier. A *universal generalization* is a formula that begins with a left parenthesis followed by a universal quantifier. An *existential generalization* is a formula that begins with a left parenthesis followed by an existential quantifier.

An *atomic formula* is one formed solely by formula formation clause {i} or {ii}. Put another way, an atomic formula is one in which no sentential connectives or quantifiers occur. A *molecular formula* is one that is not atomic. Thus a molecular formula has at least one occurrence of either a sentential connective or a quantifier. *(Revised from sentential logic.)*

A *prime formula* is a formula that is either an atomic formula or a quantified formula. A *non-prime formula* is one that is not prime. (Note that this is not entirely standard terminology. It has been used this way by some authors, but not often.)

The *main operator* of a molecular formula is the last occurrence of a sentential connective or quantifier added when the formula was constructed according to the rules above. If the main operator is a sentential connective, then it is also called the 'main connective' (as was done in the sentential language ). However, there is a change as we move to . In predicate logic, it is no longer true that all molecular formulae have a main connective. Some main operators are now quantifiers rather than sentential connectives.

## Examples[edit | edit source]

#### Example 1[edit | edit source]

By clause (i) in the definition of 'term', and are terms. Similarly, , , and are terms by clause (ii) of the definition of 'term'.

Next, by clause (iii) of the definition of 'term', the following two expressions are terms.

Then, by clause (iii) of the definition of 'term', the following is a term.

Finally, (1) is a term by clause (iii) of the definition of 'term'. However, because it contains variables, it is not a name.

#### Example 2[edit | edit source]

We already saw that (1) is a term. Thus, by clause (ii) of the definition of formula, (2) is a formula.

#### Example 3[edit | edit source]

By clause (i) in the definition of 'term', , , and are terms.

By clause (ii) of the definition for 'formula', the following are formulae.

By clause (iii-d) of the definition for 'formula', the following is a formula.

By clause (iv-a) of the definition for 'formula', the following is a formula.

By clause (iii-b) of the definition for 'formula', the following is a formula.

Finally, by clause (iv-b) of the definition for 'formula', (3) is a formula.