At least since Descartes the epistemological turn derived its impetus from the sceptical challenge to provide a justification for all knowledge claims. According to a foundational view, a claim to know something is justified only when the justification refers to ultimate grounds in the form of self?supporting propositions. This paper's title suggests that justification may be seen from a different perspective, namely that of acting. Wittgenstein's examples show that the sceptic's maxim ? doubt everything ? breaks down because some beliefs (...) and judgments have to be accepted unconditionally if one is to believe or judge something else. Wittgenstein concludes that the certainty we are looking for is to be found in the very nature of the language?game. (shrink)