Karl Popper's The Logic of Scientific Discovery has been called one of the most important philosophical works of the twentieth century. Popper discussed the problem of induction (how to justify inductive inferences) and the demarcation problem; that is, the question of how we decide which theories are scientific and which are not. In the early chapters he considered and criticised the idea that science proceeds by using the experimental results of particular tests to make general conclusions about laws (induction), moving on later in the book to propose his alternative (and solution to the demarcation problem); falsification. According to Popper, what makes a theory scientific is that it can be wrong: we can specify an experiment that, if unsuccessful, would lead us to reject the theory. In Conjectures and Refutations, a more accessible work for a general audience, several essays by Popper expanedd upon his thinking. By making bold conjectures - "sticking our necks out" - and in turn trying to refute - "falsify" - them, our knowledge of the world grows.