Ever since the term 'conspiracy theory' was first popularised by the philosopher Sir Karl Popper in the 1950s, conspiracy theories have had a bad reputation. To call a theory 'a conspiracy theory' is to imply that it is false, and that the people who believe it or who would like to investigate it (i.e. 'conspiracy theorists') are irrational. Conspiracy theories are widely held to be not only false and the products of irrationality, but also to be particularly harmful; hence they are widely thought of as a problem, which might be solved, or at least mitigated, through the intervention of social scientists, psychologists, and even philosophers. The problem is typically thought of as being of quite recent origin, or at any rate, one which has recently been getting worse, especially in the age of COVID-19 when conspiracy theories are routinely spoken of as a threat to public safety and even to democracy itself.I think all of this is mistaken. Conspiracy theories are not a new or growing problem. In fact, conspiracy theories, as such, are not a problem at all. Contrary to conventional wisdom we do not have a problem with conspiracy theories; we do, however, have a problem with the term 'conspiracy theory', along with related terms, such as 'conspiracy theorist', 'conspiracism', and 'conspiracist ideation', and this problem really is quite new and increasingly widespread. The emergence and spread of a term has been conflated with the emergence and spread of a phenomenon to which that term putatively refers. As a society we have made a use-mention error; the spread of a piece of language has wrongly been taken for the spread of something corresponding to it in the world.The bad reputation of conspiracy theories is puzzling. After all, people do conspire. That is, they engage in secretive collective behaviour which is illegal or morally questionable. Conspiracies are common in all societies throughout history, and have always been particularly common in politics. Most people conspire some of the time, and some people (e.g. spies) conspire almost all the time. Since people conspire, there can't be anything wrong with believing they conspire, hence there can't be anything wrong with believing conspiracy theories. Thinking of conspiracy theories as characteristically false and irrational is like thinking of scientific theories in this way. It is as if we thought of phrenology as a paradigm of a scientific theory. Conspiracy theories, like scientific theories, and virtually any other category of theory, are sometimes true, sometimes false, sometimes believed on rational grounds, sometimes not.It will be objected at this point that I am assuming that a conspiracy theory is simply a theory according to which a conspiracy has taken place. While this may be the most straightforward way of understanding the term 'conspiracy theory', it is not the only way. In fact the literature contains a wide array of mutually contradictory definitions. I have considered many of these alternative definitions elsewhere and concluded that none of...