Descartes claimed that the Cogito is ‘so firm and sure that all the most extravagant suppositions of the sceptics were incapable of shaking it’. This paper aims to demonstrate that this claim is false by presenting a sceptical scenario for the Cogito. It is argued that the story ‘The Circular Ruins’ by J. L. Borges illustrates that one can doubt one’s own existence and that pace Descartes (and many others) the claim ‘I am, I exist, is necessarily true whenever it (...) is put forward by me or conceived in my mind’ is false. (shrink)