This phrase is not unknown to any of us. Its ubiquity is cheeky.
Especially in the status messages. Its funny that the french form is slightly lesser known than its latin counterpart 'Cogito ergo sum' or the english translation 'I think, therefore I am'. I say its funny because Discartes (the author) wrote this originally in french for the lay people. He usually wrote in latin, but the work which contains this part was written in french to keep it nearer to the masses. The lay people now remember the latin part more. Funny how things turn out.
Anyways, was reading some Descartes lately and found out more about this phrase. The author is trying to build a model for philosphy as it exists for mathematics. He wants to be able to reach to a truth the way mathematicians reach to a conclusion. He wants to build a process for this. He calls this process meditation.
It begins with rejecting all ideas, thoughts etc which are even slighlty doubtful. One doesn't even trust one's senses as one knows they can be deceived. Also it is assumed that theres an all powerful deceiver out there whose sole purpose is to deceive you. Any thought or idea, which is even slightly dubitable is rejected.
While rejecting all such ideas. Descartes reached finally to just one final truth. That he is thinking, and so he exists. Then he build a five stage model to further this thought, the next step of which asserts God exists and then continues towards the world.
Thus was born, Je pense donc je suis.
