Cogito ergo sum[a] is a Latin philosophical proposition by René Descartes usually translated into English as "I think, therefore I am". The phrase originally appeared in French as je pense, donc je suis in his Discourse on the Method, so as to reach a wider audience than Latin would have allowed. It appeared in Latin in his later Principles of Philosophy. As Descartes explained, We cannot doubt of our existence while we doubt. A fuller form, dubito, ergo cogito, ergo sum ("I doubt, the
The evil demon, sometimes referred to as the evil genius, is a concept in Cartesian philosophy. In his 1641 Meditations on First Philosophy, René Descartes hypothesises the existence of an evil demon, a personification who is "as clever and deceitful as he is powerful, who has directed his entire effort to misleading me." The evil demon presents a complete illusion of an external world, including other minds, to Descartes' senses, where in fact there is no such external world in existence.