‘Defending knowledge’ considers some of the different philosophical responses to the problem of radical scepticism. It begins with the natural response to a philosophical puzzle: to insist on our commonsense principles and work back from there, focusing on the work of G.E. Moore. It then looks at a different kind of response to the sceptical problem, which involves the idea that perhaps there is some sort of context-shift in play in the sceptical reasoning. Finally, it discusses a more radical approach to the problem of radical scepticism outlined by Ludwig Wittgenstein. There is no one particular solution to radical scepticism, but numerous philosophical responses to the problem.