9/11 was an inside job. The Holocaust is a myth promoted to serve Jewish interests. The shootings at Sandy Hook Elementary School were a false flag operation. Climate change is a hoax perpetrated by the Chinese government. These are all conspiracy theories. A glance online or at bestseller lists reveals how popular some of them are. Even if there is plenty of evidence to disprove them, people persist in propagating them. Why? Philosopher Quassim Cassam explains how conspiracy theories are different from ordinary theories about conspiracies. He argues that conspiracy theories are forms of propaganda and their function is to promote a political agenda. Although conspiracy theories are sometimes defended on the grounds that they uncover evidence of bad behaviour by political leaders, they do much more harm than good, with some resulting in the deaths of large numbers of people. There can be no clearer indication that something has gone wrong with our intellectual and political culture than the fact that conspiracy theories have become mainstream. When they are dangerous, we cannot afford to ignore them. At the same time, refuting them by rational argument is difficult because conspiracy theorists discount or reject evidence that disproves their theories. As conspiracy theories are so often smokescreens for political ends, we need to come up with political as well as intellectual responses if we are to have any hope of defeating them.