Missing evidence: open and closed worlds

world open.
p1:Paper.
p2:Paper.
p1[reviewed -> true].
?Paper:NeedsReview <-
    ?Paper:Paper AND NOT ?Paper[reviewed -> true].
?- p2:NeedsReview.
?- ?Paper:Paper AND unknown { ?Paper[reviewed -> true] }.
expect unknown: p2:NeedsReview.

Paper p2 has no review evidence. The first answer is unknown. The second query returns p2 as a positively identified Paper whose review statement is unknown. Its candidate domain is explicit; the engine is not enumerating every imaginable object.

NOT does not turn unknown into true. The rule therefore does not derive NeedsReview for p2 in this open-world program. A scope warning points out that absence cannot be established. This is intentional evidence about the modeling assumption, not a reason to suppress the warning.

Three outcomes

True means supported under the program. False means refuted under its closed-scope semantics. Unknown means the program cannot settle the statement under its current assumptions. expect unknown checks that last outcome; it does not assert a negative fact.

A stored boolean false is also distinct: a property with value false is an ordinary assertion about that value, not automatically the negation of the same property with value true.

See also World Assumption.

Try it

Replace the world declaration with world closed. in a copy. Predict both queries before running. The NeedsReview answer becomes true; the unknown-filter query has no p2 binding.