A knowledge base KB is complete if there is no formular α such that KB ⊭ α and KB ⊭ ¬α. Example of knowledge base with incomplete knowledge: KB := { A ∨ B } Then we have KB ⊭ A and KB ⊭ ¬A. In some cases, you can make a consistent knowledge base complete with the closed world assumption - that is, adding all not-entailed literals as negations to the knowledge base. In the above example though, this would not work because it would make the knowledge base inconsistent: KB' = { A ∨ B, ¬A, ¬B } See also: * Vivid knowledge