This thesis proposes an explanation for origin of conspiracy theories about the French Revolution. Despite being unfounded, the conspiracy theory blaming the Illuminati for the Revolution became widespread because it appeared to be supported by evidence. The primary source material consists of texts produced by two contemporaries. Augustin Barruel, a counterrevolutionary Catholic Priest, and Nicolas Bonneville, a revolutionary Freemason, both alleged that Freemasons and the Illuminati had caused the Revolution of 1789. The abbé Barruel’s book Memoirs pour servir à l’histoire du jacobinisme has been a touchstone for descriptions of conspiracy theories. Bonneville’s writings are a missing link between reality and Barruel’s conspiracy theories. Bonneville claimed that a Masonic conspiracy called the “Cercle social” had caused the Revolution and continued to spread revolutionary ideology internationally through Masonic lodges. However, the Cercle social was entirely fictional. Some previously accepted evidence of actual Masonic or Illuminati involvement in the Revolution should therefore be rejected. The results do not support the idea that conspiracy theories have a shared cause, or conspiracy theories have different causes than other theories. This conclusion is further supported using explanations for other conspiracy theories, including Bonneville’s belief that Jesuits had infiltrated Freemasonry. While irrational paranoia explains Bonneville’s conspiracy theory, there is no reason to generalise this conclusion. This thesis also argues that some conspiracy theories do have recurring features with shared causes, e.g. ones caused by overreactions to signs of infiltration or subversion. Some recurring features of conspiracy theories are recurring because they provide logical consistency or prevent falsification. Despite these trends and tendencies, this thesis argues that the category itself is unsuitable as an analytical category.