Derivation (logic)

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A derivation , derivation , or deduction in logic is the extraction of statements from other statements. In doing so, final rules are applied to premises in order to arrive at conclusions . Which final rules are allowed is determined by the calculation used .

The derivation, together with the semantic inference, is one of the two logical methods of arriving at the conclusion.

Example: propositional and predicate logic

The sequence calculus deals with the derivation of sequences of the shape with the help of the sequence rules . As an illustration, we take the derivation of the sentence about the excluded third party . The rules used are described in the rules of the sequential calculus of the first-order predicate logic .

The following new sequence rule was thus derived:

It can now be used just like the basic rules of calculus.

The derivability relation and the derivability operator


To formalize the derivability is often derivative operator (also inference) is used which on the derivation relationship (also Inferenzrelation ) is defined.

If - according to the rules of a concrete calculus - the expression (the conclusion or the consequence) can be derived from the set (the premises) in finitely many steps, one writes for it ; here is the derivative relation .

This derivability relation (also inferential relation ) is a relation between a set of statements, the premises, and a single statement, the conclusion. is to be read as: " can be derived from ".

If one adds all expressions which can be derived from to a given set of expressions (one says that one forms the deductive closure), then the derivative operator (also inference operation ) is defined:

Different logics each define a different concept of derivability. There is a propositional concept of derivability, a predicate logic , an intuitionistic , a modal logic , etc.

Properties of derivative operators

There are a number of properties that most of the derivability relations (at least those mentioned above) have in common

  • Inclusion: (Every assumption is also a consequence).
  • Idempotence: if and , then (adding inferences to the assumptions does not lead to new conclusions.)
  • Monotony : If , then (adding assumptions preserves the consequences possible so far.)
  • Compactness ; If so , then there is a finite set with such that . (Every conclusion from an infinite set of assumptions can already be reached from a finite subset .)

From the first three of these properties it can be deduced that an envelope operator is; H. an extensive , monotonous , idempotent mapping .

References and comments

  1. An example of a definition is given by Kruse and Borgelt (2008) on p. 8.