Another division that is sometimes made is between overt consent and tacit consent. An oligopoly may engage in collusion, either tacit or overt, and thereby exercise market power. The opposite of tacit knowledge is explicit knowledge. Codification can therefore refer to both tacit and explicit knowledge. The knowledge of the stakeholder is both tacit and explicit. Two distinct forms of knowledge, explicit and tacit, are significant in this respect. Hobbes was a social contractarian and believed that the law had peoples' tacit consent. Kennedy received Lombardi's tacit refusal of the request. The party claiming the existence of a tacit term must formulate it clearly and precisely. The doctrine, however, met with tacit British approval. The two have a mutual respect and tacit friendship. The key to acquiring tacit knowledge is experience. Effective transfer of tacit knowledge generally requires extensive personal contact, regular interaction and trust. Sixtus gave tacit support to the conspirators. It was while writing "Personal Knowledge" that he identified the "structure of tacit knowing".