- Gödel 3-valued Logic
is a 3-valued logic, with values T, F, and N. It features a classical-like negation, and -like conditional.
Semantics
Truth Values
Common labels for the values include:
T |
1 |
just true |
N |
0.5 |
neither true nor false |
F |
0 |
just false |
Designated Values
The set of designated values for is the singleton: { T }
Truth Tables
The value of a sentence with a truth-functional operator is determined by the values of its operands according to the following tables.
| Negation | |||
|---|---|---|---|
| ¬ | |||
| T | F | ||
| N | F | ||
| F | T | ||
| Conjunction | |||
|---|---|---|---|
| ∧ | T | N | F |
| T | T | N | F |
| N | N | N | F |
| F | F | F | F |
| Disjunction | |||
|---|---|---|---|
| ∨ | T | N | F |
| T | T | T | T |
| N | T | N | N |
| F | T | N | F |
| Conditional | |||
|---|---|---|---|
| → | T | N | F |
| T | T | N | F |
| N | T | T | F |
| F | T | T | T |
Defined Operators
The Biconditional ↔, in turn, is defined in the usual way:
| Biconditional | |||
|---|---|---|---|
| ↔ | T | N | F |
| T | T | N | F |
| N | N | T | F |
| F | F | F | T |
The Material Conditional ⊃ is definable in terms of disjunction:
Likewise the Material Biconditional ≡ is defined in terms of ⊃ and ∧:
| Material Conditional | |||
|---|---|---|---|
| ⊃ | T | N | F |
| T | T | N | F |
| N | T | N | F |
| F | T | T | T |
| Material Biconditional | |||
|---|---|---|---|
| ≡ | T | N | F |
| T | T | N | F |
| N | N | N | F |
| F | F | F | T |
Compatibility Tables
does not have a separate Assertion operator, but we include a table and rules for it, for cross-compatibility.
| Assertion | |||
|---|---|---|---|
| ⚬ | |||
| T | T | ||
| N | N | ||
| F | F | ||
Predication
A sentence with n-ary predicate \(P\) over parameters has the value:
T iff is in the extension of \(P\) and not in the anti-extension of \(P\).
F iff is in the anti-extension of \(P\) and not in the extension of \(P\).
N iff is in neither the extension nor the anti-extension of \(P\).
Quantification
Existential
The value of an existential sentence is the maximum value of the sentences that result from replacing each constant for the quantified variable. The ordering of the values from least to greatest is: F, N, B, T.
Universal
The value of an universal sentence is the minimum value of the sentences that result from replacing each constant for the quantified variable. The ordering of the values from least to greatest is: F, N, B, T.
Consequence
Logical Consequence is defined in terms of the set of designated values { T }:
Logical Consequence
C is a Logical Consequence of A iff all models where A has a desginated value are models where C also has a designated value.
Tableaux
tableaux are built similary to .
Nodes
Nodes for many-value tableaux consiste of a sentence plus a designation marker: ⊕ for designated, and ⊖ for undesignated.
Trunk
To build the trunk for an argument, add a designated node for each premise, and an undesignated node for the conclusion.
Closure
Rules
In general, rules for connectives consist of four rules per connective: a designated rule, an undesignated rule, a negated designated rule, and a negated undesignated rule. The special case of negation has a total of two rules which apply to double negation only, one designated rule, and one undesignated rule.
Operator Rules
¬ Rules
∧ Rules
∨ Rules
⊃ Rules
≡ Rules
→ Rules
↔ Rules
Quantifier Rules
∃ Rules
∀ Rules
Compatibility Rules
⚬ Rules
Notes
References
Rescher, Nicholas. Many-valued Logic. United Kingdom, McGraw-Hill, 1969.
Futher Reading
Heyting, Arend. Intuitionism: An Introduction. Netherlands, North-Holland, 1966.