In his famous Discourse on Method the renowned philosopher Ren Descartes originally said in French: "Je pense, donc je suis". This later became popular in its Latin version as "Cogito ergo sum", or, "I Think, Therefore I am."
"after considering everything very thoroughly, I must finally conclude that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." (AT VII 25; CSM II 16-17)
This means that he is allowing credibility to only his consciousness and cognitive powers, and not to that of any other objective audience. Indeed his method leaves no room for the derivation of a third-person truth, which would require the acknowledgement of premises beyond the perceiving being's conscious self.
Descartes arrives into this insight into the nature of existence after undergoing the three 'waves' of doubt. The first of these brings up the fact that knowledge based on sensory inputs has not always been shown up as reliable. The second is a doubt that all that we experience and feel, indeed, our very existence itself maybe only a dream, because a lot of our thoughts that take place while dreaming, are not in fact real, but they are very similar to our waking thoughts. The third, and the most diabolical one of them all, is that we may be the subject of deception by an evil demonic force that presents to us as irrefutable knowledge, that which is not true at all.
The conclusion of "I think, Therefore I am", has been criticized on various grounds. ...