P3 - Post 3-valued Logic

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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 ... AnB 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