One of the most challenging and frustrating questions for scientists is how to combat denialism: Holocaust denial, global warming denial, anti-vaxxers, 9/11 conspiracism, creationism and more. Debunking denialist claims is essential — yet also rarely effective. We are now living in a world where even the most apparently basic truths are routinely contested. In this talk, Keith Kahn-Harris, author of Denial: The Unspeakable Truth, argues that to really understand denialism, we have to face up to what he calls 'the deniers alternative'. Denialism emerges when this alternative is so 'unspeakable' that denialism becomes a preferable option. In thinking about how to combat denialism, we also have to consider whether a world without it might not be a truth-filled utopia, but something even worse. Dr Keith Kahn-Harris is a sociologist and writer. Denial: The Unspeakable Truth is his fifth book. His badly-designed website can be found at http://www.kahn-harris.org and he tweets irregularly at https://twitter.com/KeithKahnHarris.