FDE - First Degree Entailment

FDE is a 4-valued relevance logic logic with values T, F, N and B.


Semantics

Truth Values

Common labels for the values include:

T

1

just true

N

0.25

neither true nor false

B

0.75

both true and false

F

0

just false

Designated Values

The set of designated values for FDE is: { T, B }

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
B B
N N
F T
Conjunction
T B N F
T T B N F
B B B N F
N N N N F
F F F F F
Disjunction
T B N F
T T T T T
B T B B B
N T B N N
F T B N F

Defined Operators

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 B N F
T T B N F
B T B B B
N T B N N
F T T T T
Material Biconditional
T B N F
T T B N F
B B B B B
N N B N N
F F B N T

Compatibility Tables

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

Assertion
T T
B B
N N
F F
Conditional
T B N F
T T B N F
B T B B B
N T B N N
F T T T T
Biconditional
T B N F
T T B N F
B B B B B
N N B N N
F F B N T

Predication

Predicated sentences in a model are interpreted via a predicate's extension and anti-extension.

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 \(P\).

  • B iff \(\langle a_0, ... ,a_n\rangle\) is in both the extension and anti-extension of \(P\).

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

Note, for FDE, there is no exclusivity nor exhaustion constraint on a predicate's extension and anti-extension. This means that \(\langle a_0, ... ,a_n\rangle\) could be in neither the extension nor the anti-extension of a predicate, or it could be in both the extension and the anti-extension.

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, B:

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

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

A branch is closed iff the same sentence appears on both a designated node, and undesignated node.

Designation Closure[source]
A
A

This allows for both a sentence and its negation to appear as designated on an open branch (or both as undesignated).

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
¬¬Double Negation Undesignated[source]
¬¬A
A
∧ Rules
Conjunction Designated[source]
A ∧ B
A
B
Conjunction Undesignated[source]
A ∧ B
A
B
¬Conjunction Negated Designated[source]
¬(A ∧ B)
¬A
¬B
¬Conjunction Negated Undesignated[source]
¬(A ∧ B)
¬A
¬B
∨ Rules
Disjunction Designated[source]
A ∨ B
A
B
Disjunction Undesignated[source]
A ∨ B
A
B
¬Disjunction Negated Designated[source]
¬(A ∨ B)
¬A
¬B
¬Disjunction 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
¬A
B
¬Material Biconditional Negated Designated[source]
¬(A ≡ B)
A
¬B
¬A
B
¬Material Biconditional Negated Undesignated[source]
¬(A ≡ B)
¬A
¬B
B
A

Quantifier Rules

∃ Rules
Existential Designated[source]
∃xFx
Fa
Existential Undesignated[source]
∃xFx
Fa
¬Existential Negated Designated[source]
¬∃xFx
∀x¬Fx
¬Existential Negated Undesignated[source]
¬∃xFx
∀x¬Fx
∀ Rules
Universal Designated[source]
∀xFx
Fa
Universal Undesignated[source]
∀xFx
Fa
¬Universal Negated Designated[source]
¬∀xFx
∃x¬Fx
¬Universal Negated Undesignated[source]
¬∀xFx
∃x¬Fx

Compatibility Rules

⚬ Rules
Assertion Designated[source]
⚬A
A
Assertion Undesignated[source]
⚬A
A
¬Assertion Negated Designated[source]
¬⚬A
¬A
¬Assertion 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
¬A
B
¬Biconditional Negated Designated[source]
¬(A ↔ B)
A
¬B
¬A
B
¬Biconditional Negated Undesignated[source]
¬(A ↔ B)
¬A
¬B
B
A

Notes

Some notable features of FDE include:

References

For futher reading see: