# Formal Logic/Sentential Logic/Formal Syntax

← The Sentential Language |
↑ Sentential Logic |
Informal Conventions → |

# Formal Syntax[edit | edit source]

In The Sentential Language, we informally described our sentential language. Here we give its formal syntax or grammar. We will call our language .

## Vocabulary[edit | edit source]

*Sentence letters*: Capital letters 'A' – 'Z', each with (1) a superscript '0' and (2) a natural number subscript. (The*natural numbers*are the set of positive integers and zero.) Thus the sentence letters are:

*Sentential connectives*:

- Grouping symbols:

The superscripts on sentence letters are not important until we get to the predicate logic, so we won't really worry about those here. The subscripts on sentence letters are to ensure an infinite supply of sentence letters. On the next page, we will abbreviate away most superscripts and subscripts.

## Expressions[edit | edit source]

Any string of characters from the vocabulary is an *expression* of . Some expressions are grammatically correct. Some are as incorrect in as 'Over talks David Mary the' is in English. Still other expressions are as hopelessly ill-formed in as 'jmr.ovn asgj as;lnre' is in English.

We call a grammatically correct expression of a well-formed formula. When we get to Predicate Logic, we will find that only some well formed formulas are sentences. For now though, we consider every well formed formula to be a sentence.

## Construction rules[edit | edit source]

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

- The expression consists of a single sentence letter

- The expression is constructed from other well-formed formulae and in one of the following ways:

In general, we will use 'formula' as shorthand for 'well-formed formula'. Since all formulae in are sentences, we will use 'formula' and 'sentence' interchangeably.

## Quoting convention[edit | edit source]

We will take expressions of to be self-quoting and so regard

to include implicit quotation marks. However, something like

requires special consideration. It is not itself an expression of since and are not in the vocabulary of . Rather they are used as variables in English which range over expressions of . Such a variable is called a *metavariable*, and an expression using a mix of vocabulary from and metavariables is called a *metalogical expression*. Suppose we let be and be Then (1) becomes

- '' '' ''

which is not what we want. Instead we take (1) to mean (using explicit quotes):

- the expression consisting of '' followed by followed by '' followed by followed by '' .

Explicit quotes following this convention are called *Quine quotes* or *corner quotes*. Our corner quotes will be implicit.

## Additional terminology[edit | edit source]

We introduce (or, in some cases, repeat) some useful syntactic terminology.

- We distinguish between an expression (or a formula) and an
*occurrence*of an expression (or formula). The formula

is the same formula no matter how many times it is written. However, it contains three occurrences of the sentence letter and two occurrences of the sentential connective .

- is a
*subformula*of if and only if and are both formulae and contains an occurrence of . is a*proper subformula*of if and only if (i) is a subformula of and (ii) is not the same formula as .

- An
*atomic formula*or*atomic sentence*is one consisting solely of a sentence letter. Or put the other way around, it is a formula with no sentential connectives. A*molecular formula*or*molecular sentence*is one which contains at least one occurrence of a sentential connective.

- The
*main connective*of a molecular formula is the last occurrence of a connective added when the formula was constructed according to the rules above.

- A
*negation*is a formula of the form where is a formula.

- A
*conjunction*is a formula of the form where and are both formulae. In this case, and are both*conjuncts*.

- A
*disjunction*is a formula of the form where and are both formulae. In this case, and are both*disjuncts*.

- A
*conditional*is a formula of the form where and are both formulae. In this case, is the*antecedent*, and is the*consequent*. The*converse*of is . The*contrapositive*of is .

- A
*biconditional*is a formula of the form where and are both formulae.

## Examples[edit | edit source]

By rule (i), all sentence letters, including

are formulae. By rule (ii-a), then, the negation

is also a formula. Then by rules (ii-c) and (ii-b), we get the disjunction and conjunction

as formulae. Applying rule (ii-a) again, we get the negation

as a formula. Finally, rule (ii-c) generates the conditional of (1), so it too is a formula.

This appears to be generated by rule (ii-c) from

The second of these is a formula by rule (i). But what about the first? It would have to be generated by rule (ii-b) from

But

cannot be generated by rule (ii-a). So (2) is not a formula.

← The Sentential Language |
↑ Sentential Logic |
Informal Conventions → |