An infinite-valued semantics for logic programs with negation

We give a purely model-theoretic (denotational) characterization of the semantics of logic programs with negation allowed in clause bodies. In our semantics (the first of its kind) the meaning of a program is, as in the classical case, the unique minimum model in a program-independent ordering. We use an expanded truth domain that has an uncountable linearly ordered set of truth values between False (the minimum element) and True (the maximum), with a Zero element in the middle. The truth values below Zero are ordered like the countable ordinals. The values above Zero have exactly the reverse order. Negation is interpreted as reflection about Zero followed by a step towards Zero; the only truth value that remains unaffected by negation is Zero. We show that every program has a unique minimum model M-P, and that this model can be constructed with a T-P iteration which proceeds through the countable ordinals. Furthermore, collapsing the true and false values of the infinite-valued model M-P to (the classical) True and False, gives a three-valued model identical to the well-founded one.