Deep inference names a general idea in structural proof theory that breaks with the classical sequent calculus by generalising the notion of structure to permit inference to occur in contexts of high structural complexity. The term ''deep inference'' is generally reserved for proof calculi where the structural complexity is unbounded; in this article we will use non-shallow inference to refer to calculi that have structural complexity greater than the sequent calculus, but not unboundedly so, although this is not at present established terminology. Deep inference is not important in logic outside of structural proof theory, since the phenomena that lead to the proposal of

formal system A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system. A for ...

s with deep inference are all related to the

cut-elimination theorem The cut-elimination theorem (or Gentzen's ''Hauptsatz'') is the central result establishing the significance of the sequent calculus. It was originally proved by Gerhard Gentzen in his landmark 1934 paper "Investigations in Logical Deduction" fo ...

. The first calculus of deep inference was proposed by Kurt Schütte,Kurt Schütte. Proof Theory. Springer-Verlag, 1977. but the idea did not generate much interest at the time. Nuel Belnap proposed display logic in an attempt to characterise the essence of structural proof theory. The

calculus of structures The calculus of structures is a proof calculus with deep inference for studying the structural proof theory of noncommutative logic. The calculus has since been applied to study linear logic, classical logic, modal logic, and process calculi, and m ...

was proposed in order to give a cut-free characterisation of

noncommutative logic Noncommutative logic is an extension of linear logic which combines the commutative connectives of linear logic with the noncommutative multiplicative connectives of the Lambek calculus. Its sequent calculus relies on the structure of order varietie ...

. Cirquent calculus was developed as a system of deep inference allowing to explicitly account for the possibility of subcomponent-sharing.

Notes

Further reading