The essay comments Descartes’ Meditations I. Starting from the suggestion that the ‘material’ modes of the Pyrrhonists can be distinguished from the ‘formal’ modes of the Academics, the text is read as a sequence of reasons for doubting whole sets of beliefs. These operations are ‘formal’ insofar as Descartes’ meditator recognises that he cannot enumerate one by one the members of these sets. First, he recalls how many beliefs he formed in infancy were erroneous, and identifies one source of error in their coming on the authority of others. He then notices that, even in favourable conditions, he could form false beliefs, for instance if he were suffering from persistent delusions. Favourable conditions cannot be delimited unless one knows o...