© 2020 Elsevier Inc.The present work introduces the Knowledge Harnessing, by showing its theoretical foundations as well as a three-way decision model to deal with it. The problem poses how to extract valid information about a specific context from conflicting or uncertain information received by a system (or agent). With this aim, forgetting variable operators are used to both characterize the problem from the logical point of view and provide a theoretical solution as an acceptance-rejection problem. Since the formalization is semantic in nature (it considers the models of the knowledge base that admit the extracted knowledge), general bounds are provided for acceptance-rejection evaluation on boundary region.