Descartes claims that his existence of a thinking thing is the one certain truth, which can serve as the foundation for all other knowledge. With nothing more than just the knowledge that he is a thinking thing, Descartes goes on to further meditate on more monumental topics. Not only does this realization of his existence serve as the foundation for all other knowledge, it served as the starting point of a new era of modern western philosophy. Although the idea of existence of a thinking thing is a stepping stone to further proceed in his argument, Descartes leaves room for no further doubt and systematically explains his reasoning.
To further probe at what Descartes means by his certain true, one must know how he came to this conclusion. He is firm in his resolve to continue his search for certainty and to discard as false anything that is open to the slightest doubt.
Remembering Archimedes' famous saying that he could shift the entire earth given one immovable point, Descartes searched for one certain thing that he could be completely certain. He supposes that what he sees does not exist, that his memory is faulty, that he has no senses and no body, that extension, movement and place are mistaken notions. Then, he wonders, is not he, the source of these meditations, not something? He has conceded that he has no senses and no body, but does that mean he cannot exist either? He has also noted that the physical world does not exist, which might also seem to imply his nonexistence, but yet to have these doubts, he must exist. For an evil demon to mislead him in all these insidious ways, he must exist in order to be misled. There must be an "I" that can doubt, be deceived,