Belnaps four-valued logic (short:) is a logical system with four truth values, which enables paraconsistent reasoning. It was developed in 1975 by Nuel D. Belnap . Belnaps tetravalent logic used in contrast to classical logic four logical values: , , and . In this system, conclusions can also be drawn from classically logically inconsistent sets.
Paraconsistent reasoning
A consequence operation is called paraconsistent if the entire logical language cannot be deduced from an inconsistent set of propositional formulas. Meaningful, logical conclusions from inconsistent formula sets are possible.
Let a propositional language with a signature be a set of formulas of this language. A consequence relation is called para consistent if there is a set of formulas from there, so not that
applies.
Truth values
In contrast to classical logic , which only knows the truth values and , Belnaps four-valued logic is based on two further truth values and . expresses inconsistency, i.e. an excess of knowledge. however, describes the lack of knowledge, also referred to as incomplete .
Truth value
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representation
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(1, 0)
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(0, 1)
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(1, 1)
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(0, 0)
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Analogous to classical logic, these values are represented with the help of numbers.
Two comparison relations are defined on the basis of the four truth values.
compares two values regarding their truthfulness,
compares the knowledge content.
Comparisons of two truth values using these relations are defined by:
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iff. and .
-
iff. and .
So is and . The values and are incomparable with regard to , are analogous and are incomparable with regard to .
evaluation
The evaluation function is defined by
and provides interpretations for atomic logical formulas.
Junctures
In addition to interpretations for atomic formulas, evaluations of the logical joiners , and , and for (strong implication ) are specified recursively .
Let A and B be formulas.
and
-
.
In addition, derived connectives are defined, similar to the propositional material implication:
With the help of the interpretation function , logical expressions in Belnap's four-valued logic can be evaluated by assigning a truth value to each atomic formula and interpreting the formulas recursively.
Truth tables
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Strong implication
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Material implication
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implication
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Fulfillment
Two values from are interpreted as true and combined into a set of designated values:
An interpretation satisfies a formula ,
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,
if applies
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.
It is also said to be a model of . The set of all models of a set of propositional formulas is called.
Inference
As in classical propositional logic, an inference relation is defined by means of which new knowledge can be inferred from existing knowledge.
Be , a lot of -formulae, or a formula.
applies if every -Model of is also a -Model of , i.e. if
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.
The consequence relation is monotonous , compact and paraconsistent .
properties
The logical system has similar properties to classical propositional logic.
De Morgan's rule
cut
As in propositional logic:
Tautologies
In no tautologies exist. In particular is
no tautology.
Material implication
As defined in propositional logic can also be in the material implication
be used. However, it loses its strength and formula amount
can be designated (true) even if B is not designated. The law of the excluded middle applies not so. The strong implication was introduced in to remedy this shortcoming.
Strong implication
There is a similar relationship between the strong implication and the inference relation as in propositional logic between and .
Let be a set of formulas; , Formulas. The following applies:
-
iff.
Examples
In the following the three atoms , and are used, which can be interpreted with the following meanings:
atom |
meaning
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can fly
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penguin
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bird
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Paraconsistency
The formula
is inconsistent in classical logic.
In, however, there are interpretations with which is designated, i.e.
Examples of such assignments are:
Strong implication
With the help of material and strong implication, different types of inference can be modeled. The material implication models conclusions with exceptions, while the strong implication models knowledge without exception.
The formula set has 6 models:
model |
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P |
V
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M1 |
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M2 |
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M3 |
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M4 |
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M5 |
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M6 |
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literature
Web links