Abstract : In chapter 5 of Knowledge and its Limits, Williamson formulates an argument against the principle (KK) of epistemic transparency, or luminosity of knowledge, namely “that if one knows something, then one knows that one knows it” (Williamson 2000: 115). Principle (KK), which corresponds to axiom schema 4 of propositional modal logic, is also called “positive introspection” and was originally defended by Hintikka in his seminal work on epistemic logic (Hintikka 1962: c. 5, “Knowing that one knows”). Williamson's argument proceeds by reductio: from the description of a situation of approximate knowledge, he shows that a contradiction can be derived on the basis of principle (KK) and additional epistemic principles that he claims are better grounded. One of them is a reflective form of the margin for error principle defended by Williamson in his account of knowledge. We argue that Williamson's reductio rests on the inappropriate identification of distinct forms of knowledge. More specifically, an important distinction between perceptual knowledge and non-perceptual knowledge is wanting in his statement and analysis of the puzzle. The (KK) principle and the margin for error principle can coexist, provided their domain of application is referred to the right sort of knowledge.