Therefore from the fact alone that I know that I exist and that, at the same time, I notice absolutely nothing else that belongs to my nature apart f… - René Descartes

I think; therfore I am.

It is a common failing of mortals to deem the more difficult the fairer.

For, occupied incessantly with the consideration of the limits prescribed to their power by nature, they [philosophers of former times] became so entirely convinced that nothing was at their disposal except their own thoughts, that this conviction was of itself sufficient to prevent their entertaining any desire of other objects; and over their thoughts they acquired a sway so absolute, that they had some ground on this account for esteeming themselves more rich and more powerful, more free and more happy, than other men who, whatever be the favors heaped on them by nature and fortune, if destitute of this philosophy, can never command the realization of all their desires.

"("I think, therefore I am"), the idea that the mere act of thinking about one's existence proves there is someone there to do the thinking."