This essay establishes some theoretical basis for the study of conspiracy theories, understood as cognitive constructions about society related, but different, from other cognitive constructions, such as magic, science, and common sense. The points of convergence and divergence between these constructions are characterized through examples taken form the work of Harvey Sacks, quoted here not as a conversation analyst, but as a social theorist. The essay concludes with the hypothesis that many conspiracy theories derive their explanatory strength from their confluence with other cognitive constructions.