The KK thesis or KK principle is a principle of epistemic logic which states that "If you know that P is the case then you know that you know that P is the case." This means that one cannot know that P if he does not know whether his knowledge of P is correct. Its application in science can be expressed in the way that it must not only justify its knowledge claims but it must also justify its method of justifying.
In formal notation the principle can be stated as: "Kp→KKp" (literally: "Knowing p implies the knowing of knowing p"). It is said that the wide acceptance of the thesis steered many philosophers of science towards skepticism since the thesis features infinite regress and that to know is interpreted as "to know with certainty that one knows". An account goes as far as saying that the thesis is false due to these reasons since any argument that depends upon it is unsatisfactory.
An application of the principle may involve Hume's skepticism, which holds that it is not possible to know the induction hypothesis needed to determine the derivative knowledge that P from what is already known. This finally leads to the Humean skeptical conclusion if it is attained using KK hypothesis.
The KK thesis has been associated with the notion of the infallibility of knowledge since ancient philosophers sometimes characterized the latter according to the former's terms. Plato's view on infallibility, for example, can be approached according to its framework, particularly concerning his position stated in Theatetus that truth can only be attained by infallibly knowing it.
Jaakko Hintikka, argued that the plausibility of the KK thesis turns upon the acceptance of a strong notion of knowledge and that it is also in part constitutive of that notion. He traced the thesis' earliest iteration in Plato's Charmides and the Book of Lambda of Aristotle's Metaphysics. He also cited examples drawn from other points of philosophical history, citing the works of Augustine, Averoes, Thomas Aquinas, and Baruch Spinoza, among others. In response to the critique about the implausibility of the KK thesis, Hintikka stated that it is not an important point because what matters is that the principle is able "to capture a strong sense of knowledge".
After Hintikka's work involving epistemic logic, the status of the KK thesis and its interpretations has been disputed.