Metalogic is the study of the metatheory of logic. While logic is the study of the manner in which logical systems can be used to decide the correctness of arguments, metalogic studies the properties of the logical systems themselves.^[1] According to Geoffrey Hunter, while logic concerns itself with the "truths of logic," metalogic concerns itself with the theory of "sentences used to express truths of logic."^[2]

The basic objects of study in metalogic are formal languages, formal systems, and their interpretations. The study of interpretation of formal systems is the branch of mathematical logic known as model theory, while the study of deductive apparatus is the branch known as proof theory.

Important distinctions in metalogic

Syntax-semantics

In metalogic, 'syntax' has to do with formal languages or formal systems without regard to any interpretation of them, whereas, 'semantics' has to do with interpretations of formal languages. The term 'syntactic' has a slightly wider scope than 'proof-theoretic', since it may be applied to properties of formal languages without any deductive systems, as well as to formal systems. 'Semantic' is synonymous with 'model-theoretic'.

Use-mention

In metalogic, the words 'use' and 'mention', in both their noun and verb forms, take on a technical sense in order to identify an important distinction.^[2] The use–mention distinction (sometimes referred to as the words-as-words distinction) is the distinction between using a word (or phrase) and mentioning it. Usually it is indicated that an expression is being mentioned rather than used by enclosing it in quotation marks, printing it in italics, or setting the expression by itself on a line. The enclosing in quotes of an expression gives us the name of an expression, for example:

'Metalogic' is the name of this article.

This article is about metalogic.

Type-token

The type-token distinction is a distinction in metalogic, that separates an abstract concept from the objects which are particular instances of the concept. For example, the particular bicycle in your garage is a token of the type of thing known as "The bicycle." Whereas, the bicycle in your garage is in a particular place at a particular time, that is not true of "the bicycle" as used in the sentence: "The bicycle has become more popular recently." This distinction is used to clarify the meaning of symbols of formal languages.

Overview

Formal language

A formal language is an organized set of symbols the essential feature of which is that it can be precisely defined in terms of just the shapes and locations of those symbols. Such a language can be defined, then, without any reference to any meanings of any of its expressions; it can exist before any formal interpretation is assigned to it -- that is, before it has any meaning. First order logic is expressed in some formal language. A formal grammar determines which symbols and sets of symbols are formulas in a formal language.

A formal language can be defined formally as a set A of strings (finite sequences) on a fixed alphabet α. Some authors, including Carnap, define the language as the ordered pair <α, A>.^[3] Carnap also requires that each element of α must occur in at least one string in A.

Formal grammar

A formal grammar (also called formation rules) is a precise description of a the well-formed formulas of a formal language. It is synonymous with the set of strings over the alphabet of the formal language which constitute well formed formulas. However, it does not describe their semantics (i.e. what they mean).

Formal systems

A formal system (also called a logical calculus, or a logical system) consists of a formal language together with a deductive apparatus (also called a deductive system). The deductive apparatus may consist of a set of transformation rules (also called inference rules) or a set of axioms, or have both. A formal system is used to derive one expression from one or more other expressions.

A formal system can be formally defined as an ordered triple <α,${\displaystyle {\mathcal {I))}$,${\displaystyle {\mathcal {D))}$d>, where ${\displaystyle {\mathcal {D))}$d is the relation of direct derivability. This relation is understood in a comprehensive sense such that the primitive sentences of the formal system are taken as directly derivable from the empty set of sentences. Direct derivability is a relation between a sentence and a finite, possibly empty set of sentences. Axioms are laid down in such a way that every first place member of ${\displaystyle {\mathcal {D))}$d is a member of ${\displaystyle {\mathcal {I))}$ and every second place member is a finite subset of ${\displaystyle {\mathcal {I))}$.

It is also possible to define a formal system using only the relation ${\displaystyle {\mathcal {D))}$d. In this way we can omit ${\displaystyle {\mathcal {I))}$, and α in the definitions of interpreted formal language, and interpreted formal system. However, this method can be more difficult to understand and work with. ^[4]

Formal proofs

A formal proof is a sequences of well-formed formulas of a formal language, the last one of which is a theorem of a formal system. The theorem is a syntactic consequence of all the well formed formulae preceding it in the proof. For a well formed formula to qualify as part of a proof, it must be the result of applying a rule of the deductive apparatus of some formal system to the previous well formed formulae in the proof sequence.

Formal interpretations

A formal interpretation of a formal system is the assignment of meanings, to the symbols, and truth-values to the sentences of the formal system. The study of formal interpretations is called Formal semantics. Giving an interpretation is synonymous with constructing a model.

Results in metalogic

Results in metalogic consist of such things as formal proofs demonstrating the consistency, comleteness, and decidability of particular formal systems.

Major results in metalogic include:

• Proof of the uncountability of the set of all subsets of the set of natural numbers (Cantor's theorem 1891)
• Proof of the consistency of truth-functional propositional logic (Emil Post 1920)
• Proof of the semantic completeness of truth-functional propositional logic
• Proof of the syntactic completeness of truth-functional propositional logic
• Proof of the decidability of truth-functional propositional logic (Post 1920)
• Proof of the consistency of first order monadic predicate logic
• Proof of the semantic completeness of first order predicate logic (Gödel's completeness theorem 1930)
• Proof of the semantic completeness of first order monadic predicate logic
• Proof of the decidability of monadic predicate logic (Leopold Löwenheim 1915)
• Proof of the consistency of first order predicate logic (Hilbert-Ackermann 1928)
• Proof of the undecidability of first order predicate logic (Church's theorem 1936)