Galen Strawson, Professor of Philosophy at the University of Oxford deliverd the 2017 Isaiah Berlin Lecture at Wolfson College. The lecture was introduced by the College President, Hermione Lee. There occurred in the twentieth century the most remarkable episode in the whole history of ideas-the whole history of human thought. A number of thinkers denied the existence of something we know with certainty to exist: consciousness, conscious experience. Others held back from the Denial, but claimed that it might be true-a claim no less remarkable than the Denial. It is instructive to document some aspects of this episode, with particular reference to the rise of philosophical behaviourism, and the (connected) rise of a conception of naturalism that transformed the doctrine of materialism from a consciousness affirming-view into a consciousness-denying view. There is then a further task: to try to explain how it is possible that intelligent human beings should come to deny the existence of something that certainly exists.