KK-thesis the thesis that knowing entails knowing that one knows, symbolized in propositional epistemic logic as Kp P KKp, where ‘K’ stands for knowing. According to the KK-thesis, the (propositional) logic of knowledge resembles the modal system S4. The KK-thesis was introduced into epistemological discussion by Hintikka in Knowledge and Belief (1962). He calls the KK- thesis a ‘virtual implication,’ a conditional whose negation is ‘indefensible.’ A tacit or an explicit acceptance of the thesis has been part of many philosophers’ views about knowledge since Plato and Aristotle. If the thesis is formalized as Kap P KaKap, where ‘Ka’ is read as ‘a knows that’, it holds only if the person a knows that he is referred to by ‘a’; this qualification is automatically satisfied for the first-person case. The validity of the thesis seems sensitive to variations in the sense of ‘know’; it has sometimes been thought to characterize a strong concept of knowledge, e.g., knowledge based on (factually) conclusive reasons, or active as opposed to implicit knowledge. If knowledge is regarded as true belief based on conclusive evidence, the KK- thesis entails that a person knows that p only if his evidence for p is also sufficient to justify the claim that he knows that p; the epistemic claim should not require additional evidence. See also EPISTEMOLOGY. R.Hi.