René Descartes showed some two hundred and fifty years ago, that our own consciousness is the one thing of which we can be absolutely certain.
Descartes, like many others before and after, wished to establish whether or not there are any absolute truths. To do this he invented his method of doubt -- a quite revolutionary approach at that time. Anything that could in any way be doubted could not, he argued, be an absolute truth.
Descartes found that he could doubt almost everything. He could doubt any theory or philosophy; he could doubt what anybody said; he could doubt what his eyes showed him of the world. He could even doubt his own thought processes. But what he could not doubt was that he was doubting. This revealed one certainty: he was thinking. If he was thinking he had to be a thinking being. As he put it, in Latin, Cogito, ergo sum -- I exist, hence I am.
These famous words are sometimes interpreted as my thinking creates my existence. This was not Descartes' intention. He was saying that his thinking proved his existence. What he had shown was that subjective experience is an undeniable fact of life. To us this may seem obvious, but at the time it was a major breakthrough.