Cogito, ergo sum (See CSMK III 159, 161.) Another predecessor was Avicenna's "Floating Man" thought experiment on human self-awareness and self-consciousness. The 8th century Hindu philosopher Adi Shankara wrote in a similar fashion, No one thinks, 'I am not', arguing that one's existence cannot be doubted, as there must be someone there to doubt. The central idea of "cogito, ergo sum" is also the topic of Mandukya Upanishad. Spanish philosopher Gómez Pereira in his 1554 work "De Inmortalitate Animae", published in 1749, wrote ""nosco me aliquid noscere, & quidquid noscit, est, ergo ego sum"" ("I know that I know something, anyone who knows
Cogito, ergo sum ("If I am mistaken, I am") (book XI, 26), and also anticipates modern refutations of the concept. Furthermore, in the "Enchiridion" Augustine attempts to refute skepticism by stating, "[B]y not positively affirming that they are alive, the skeptics ward off the appearance of error in themselves, yet they do make errors simply by showing themselves alive; one cannot err who is not alive. That we live is therefore not only true, but it is altogether certain as well" (Chapter 7 section 20). In 1640 correspondence, Descartes thanked two colleagues for drawing his attention to Augustine and notes similarity and difference.