Contributor(s): Professor John Worrall | Statements can be significant despite being “statements of the bleedin’ obvious”. The philosopher David Hume’s remark that ‘The rational man adjusts his beliefs to the evidence’ falls exactly into this category. It is surely “bleedin’ obvious” that our views (and hence our policies) ought to be based on evidence, but Hume’s claim is important exactly because it is so often ignored in practice. In these Trumpish times of disinformation and fake news, people’s views seem more and more to be based on what they would like to be true rather than on any evidence that they are in fact true. But agreeing that our views should be evidence-based is only the first step. When it comes to the details of what is involved in basing views (and policies) on evidence, things are by no means as straightforward as might be thought. This lecture investigates some of the difficulties using Evidence-Based Medicine as a test-case (though the lessons generalise to other areas such as the social sciences). John Worrall is Professor of Philosophy, LSE. He specialises in the philosophical issues raised by major theory-change in science; and is especially known for his work on structural realism. J. McKenzie Alexander is Professor of Philosophy and Head of LSE's Department of Philosophy, Logic and Scientific Method. The Department of Philosophy, Logic and Scientific Method (@LSEPhilosophy) at LSE was founded by Professor Sir Karl Popper in 1946, and remains internationally renowned for a type of philosophy that is both continuous with the sciences and socially relevant.