P3 - Post 3-valued Logic

Photo of Emil Post

Emil Post

Emil Post's three-valued logic, with values T, F, and N. It features a deviant negation that performs a cyclic shift in value.


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 P3 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 N
N F
F T
Disjunction
∨ T N F
T T T T
N T N N
F T N F

Defined Operators

Conjunction ∧ is defined in terms of ¬ and ∨ in a standard way:

A ∧ B \(:=\) ¬(¬A ∨ ¬B)

Given the behavior of ¬, however, this yields a non-standard table:

Conjunction
∧ T N F
T F F N
N F T N
F N N N

The Material Conditional ⊃ is definable in terms of disjunction:

A ⊃ B \(:=\) ¬A ∨ B

Likewise the Material Biconditional ≡ is defined in terms of ⊃ and ∧:

A ≡ B \(:=\) (A ⊃ B) ∧ (B ⊃ A)
Material Conditional
⊃ T N F
T T N N
N T N F
F T T T
Material Biconditional
≡ T N F
T F F F
N F T N
F F N F

Compatibility Tables

P3 does not have separate Assertion or Conditional operators, but we include tables and rules for them, for cross-compatibility.

Assertion
⚬
T T
N N
F F
Conditional
→ T N F
T T N N
N T N F
F T T T
Biconditional
↔ T N F
T F F F
N F T N
F F N F

Predication

A sentence with n-ary predicate \(P\) over parameters \(\langle a_0, ... ,a_n\rangle\) has the value:

  • T iff \(\langle a_0, ... ,a_n\rangle\) is in the extension of \(P\) and not in the anti-extension of \(P\).

  • F iff \(\langle a_0, ... ,a_n\rangle\) is in the anti-extension of \(P\) and not in the extension of \(P\).

  • N iff \(\langle a_0, ... ,a_n\rangle\) is in neither the extension nor the anti-extension of \(P\).

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

P3 tableaux are built similary to FDE.

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.

To build the trunk for the argument A1 ... An ∴ B write:
A1 ⊕
⋮
An ⊕
B ⊖

Closure

⊗Glut Closure[source]
A ⊕
⋮
¬A ⊕
⊗
⊗Designation Closure[source]
A ⊕
⋮
A ⊖
⊗

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
¬¬⊕Double Negation Designated[source]
¬¬A ⊕
⋮
¬A ⊖
A ⊖
¬¬⊖Double Negation Undesignated[source]
¬¬A ⊖
⋮
¬A ⊕
A ⊕
∧ Rules
∧⊕Conjunction Designated[source]
A ∧ B ⊕
⋮
¬A ⊖
A ⊖
¬B ⊖
B ⊖
∧⊖Conjunction Undesignated[source]
A ∧ B ⊖
⋮
¬A ⊕
A ⊕
B ⊕
¬B ⊕
¬∧⊕Conjunction Negated Designated[source]
¬(A ∧ B) ⊕
⋮
A ⊕
¬B ⊖
B ⊕
¬A ⊖
¬∧⊖Conjunction Negated Undesignated[source]
¬(A ∧ B) ⊖
⋮
¬A ⊖
A ⊖
¬B ⊖
B ⊖
¬A ⊕
¬B ⊕
∨ Rules
∨⊕FDEDisjunction Designated[source]
A ∨ B ⊕
⋮
A ⊕
B ⊕
∨⊖FDEDisjunction Undesignated[source]
A ∨ B ⊖
⋮
A ⊖
B ⊖
¬∨⊕FDEDisjunction Negated Designated[source]
¬(A ∨ B) ⊕
⋮
¬A ⊕
¬B ⊕
¬∨⊖FDEDisjunction Negated Undesignated[source]
¬(A ∨ B) ⊖
⋮
¬A ⊖
¬B ⊖
⊃ Rules
⊃⊕Material Conditional Designated[source]
A ⊃ B ⊕
⋮
¬A ∨ B ⊕
⊃⊖Material Conditional Undesignated[source]
A ⊃ B ⊖
⋮
¬A ∨ B ⊖
¬⊃⊕Material Conditional Negated Designated[source]
¬(A ⊃ B) ⊕
⋮
¬(¬A ∨ B) ⊕
¬⊃⊖Material Conditional Negated Undesignated[source]
¬(A ⊃ B) ⊖
⋮
¬(¬A ∨ B) ⊖
≡ Rules
≡⊕Material Biconditional Designated[source]
A ≡ B ⊕
⋮
(A ⊃ B) ∧ (B ⊃ A) ⊕
≡⊖Material Biconditional Undesignated[source]
A ≡ B ⊖
⋮
(A ⊃ B) ∧ (B ⊃ A) ⊖
¬≡⊕Material Biconditional Negated Designated[source]
¬(A ≡ B) ⊕
⋮
¬((A ⊃ B) ∧ (B ⊃ A)) ⊕
¬≡⊖Material Biconditional Negated Undesignated[source]
¬(A ≡ B) ⊖
⋮
¬((A ⊃ B) ∧ (B ⊃ A)) ⊖

Compatibility Rules

⚬ Rules
⚬⊕FDEAssertion Designated[source]
⚬A ⊕
⋮
A ⊕
⚬⊖FDEAssertion Undesignated[source]
⚬A ⊖
⋮
A ⊖
¬⚬⊕FDEAssertion Negated Designated[source]
¬⚬A ⊕
⋮
¬A ⊕
¬⚬⊖FDEAssertion Negated Undesignated[source]
¬⚬A ⊖
⋮
¬A ⊖
→ Rules
→⊕Conditional Designated[source]
A → B ⊕
⋮
¬A ∨ B ⊕
→⊖Conditional Undesignated[source]
A → B ⊖
⋮
¬A ∨ B ⊖
¬→⊕Conditional Negated Designated[source]
¬(A → B) ⊕
⋮
¬(¬A ∨ B) ⊕
¬→⊖Conditional Negated Undesignated[source]
¬(A → B) ⊖
⋮
¬(¬A ∨ B) ⊖
↔ Rules
↔⊕Biconditional Designated[source]
A ↔ B ⊕
⋮
(A → B) ∧ (B → A) ⊕
↔⊖Biconditional Undesignated[source]
A ↔ B ⊖
⋮
(A → B) ∧ (B → A) ⊖
¬↔⊕Biconditional Negated Designated[source]
¬(A ↔ B) ⊕
⋮
¬((A → B) ∧ (B → A)) ⊕
¬↔⊖Biconditional Negated Undesignated[source]
¬(A ↔ B) ⊖
⋮
¬((A → B) ∧ (B → A)) ⊖

Notes

References