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In the wake of Ernst Tugendhat’s commentary on §420 of the Zettel, which sees in this brief remark “an inversion of the entire theory of knowledge of the Modern Age,” and in particular a reversal of criticism, this contribution examines the way in which Wittgenstein highlights a presupposition common to traditional philosophies of knowledge (idealist, realist, skeptical): the epistemological priority of the language of appearances over the descriptive language of reality. Using the fable of the student prodigy and his two teachers (Zettel §409–435), Wittgenstein reminds us that, from a logical point of view, the ability to master the descriptive language of reality is primary, and that making a judgment about appearances is conditional on this mastery. He shows that modern epistemologies (idealist and realist) ignore the irreducible part played by practice in mastering our concepts, and that sense data theorists objectify appearances. The critical problem of knowledge thus gives way to the grammar of “knowing” and “doubting.”