README.md

Belnapian

Belnapian is a library that provides basic types and operations for multiple-valued logics.

3-Valued Logic

For 3-valued logics, we provide the TernaryTruth type:

pub enum TernaryTruth {
    False,
    True,
    Unknown,
}

Belnapian also provides the basic operations and, or, not and xor, but it does not provide "implication" connectives, as they differ between different 3-valued logic systems such as Kleene logic, RM3 logic, or Łukasiewicz logic.

My personal recommendation on using these "truth values" is to treat them not as the actual truth values attributed to a proposition, but to treat them as our subjective knowledge of them (hence the term "Unknown", which implies the need of a sentient being not-knowing something).

4-Valued Belnap Logic

Belnapian, unsurprisingly, also provides basic support for Belnap's 4-valued logic.

As in the case of support for 3-valued logics, this library only provides basic building blocks, and not any kind of fully fledged inference system.

pub enum Belnapian {
    // The `Neither` truth value is useful to identify propositions to
    // which we cannot assign any classical truth value. This often
    // happens when the proposition is not well-formed or when it is
    // self-contradictory.
    Neither,

    False,
    True,

    // We can understand `Both` as a superposition of `True` and
    // `False`. A natural case where it makes sense to assign this
    // truth value is when we have a proposition that, given our
    // current set of axioms, could be either `True` or `False`
    // (remember Gödel's incompleteness theorems).
    //
    // In other words, in case that a proposition (or its negation) is
    // independent of our axioms and could be added as a new axiom
    // without causing any inconsistency, then we can assign the
    // `Both` truth value to it.
    Both,
}

In constrast to the case of the 3-valued logics with an Unknown value, the Both and Neither truth values aren't necessarily tied to our subjective knowledge of the truth value for a given proposition.

Assuming that we operate with a well-known set of axioms, we could use them to talk about the "real" underlying truth value for a given proposition.

15-Valued Extended Belnap Logic (with Unknown values)

The most important feature of the Belnapian library is its support for a 15-valued logic combining Belnap's 4-valued logic with subjective unknown values.

pub enum EBelnapian {
    Known(Belnapian),
    Unknown(Unknown),
}

// The enum variants' names are ugly, but once we know what they
// represent, it becomes much easier to use & understand them.
pub enum Unknown {
    NF__, // Could be Neither or False
    N_T_,
    _FT_, // Could be False or True
    NFT_,
    N__B,
    _F_B,
    NF_B,
    __TB, // Could be True or Both
    N_TB,
    _FTB,
    NFTB,
}

Once we have more than 2 "objective" truth values, our unknowns can represent more than one set of possible values (in 3-valued logic, the Unknown value represents the set {False, True}).

Our "unknown values" represent the sets present in the power set of {Neither, False, True, Both}, except for the null set ø and the singletons {Neither}, {False}, {True}, and {Both} (that is, 2⁴-5 = 16-5 = 11 values).

The amazing aspect of these "unknown values" is that we can still apply classic logic operations to them and obtain useful results. This library relies on pre-computed tables to save you a ton of time when dealing with uncertainty in logic calculations.