In his search for unassailable knowledge, Descartes reaches in his first Meditation a hurdle in the form of his deceiving demon thought experiment. The demon Descartes imagines is one who deceives him about the authenticity of his thoughts and beliefs. Such a demon threatens to destroy Descartes’ claim to knowledge in almost all areas. The fact that in spite of the demon, Descartes cannot doubt his perceptions as they appear to him is of little consolation. It leaves him in a position no better than being able to say he is certain he feels he has a headache, even though he cannot be sure he has a head. As such Descartes’ knowledge of the physical world, if there is such a thing, is fundamentally incomplete. Though he experiences a plethora of ethereal sense data, Descartes has no facility by which to attribute to it a source.
If Descartes is rigidly to conform to his principle of methodical doubt, it is imperative that he is able to prove at the very least that his own existence is not an illusion, as all of his other beliefs could be. At this stage Descartes is interested not with proving he exists in the way he perceives himself to through his senses. He seeks simply to prove that he is an entity of some kind.
In his second Meditation, Descartes concludes that there can be no doubt that he exists in some form. He reasons that in order to be deceived by the demon he must necessarily exist. Descartes writes, ‘There is therefore no doubt that I exist, if he deceives me; and let him deceive me as much as he likes, he can never cause me to be nothing, so long as I think I am something’. Descartes reasons that it is incoherent to suggest that something that does not exist can be deceived. Simply put, there must be an object of any deception. Moreover, Descartes argues that ‘the mere fact that I thought at all’ guarantees the fact of his existence. Even if one were to entertain the thought that they did not exist, it is by this same action that they unavoidably reaffirm their existence. Just as one must exist to be deceived, one must exist to doubt that very existence. This argument has come to be known the ‘cogito’, earning its name from the phrase ‘cogito ergo sum’ meaning "I think therefore I am". It is used by Descartes in his Discourse on Method and the Meditations.