In his Discourse on Method, Descartes arrived at his conclusion by systematically doubting the reality of every belief that ultimately came through his senses. If Mindy Dog, mentioned above, was alive in Descartes time (1596-1650), he would look at her to see a black shape with fur, four feet, and an insatiable appetite for food.
But upon reflection, what he saw of Mindy, in the old Scholastic sense, were mere sensations of accidents. Mindy’s blackness, her shape, the appearance of her fur, and her hunger were all qualities that came through his senses. Descartes believed that his mind assembled these appearances into a form that he recognized as “dog.” If it wasn’t for his mind, Descartes would say that Mindy might not be out there in reality at all. She existed only within the confines of his mind.
Similarly, even scientific knowledge, because it comes to us through sensation, we cannot be certain of. This line of reasoning implies that what we know of reality exists only in mente. Thus, Descartes doubted the existence of anything and everything including his own corporeal body. But he had three additional reasons for doubt!
1) As a further argument Descartes believed, that since he often had the same perceptions while dreaming as during wakefulness, all of reality might be a dream in his mind.
2) An all powerful God could easily deceive us into believing things about reality which are not true. This would include our understanding of mathematical theories and operations.
3) Instead of deception by God, a powerful demon could deceive us. "I shall suppose, therefore, that there is, not a true God, who is the sovereign source of truth, but some evil demon, no less cunning and deceiving than powerful, who has used all his artifice to deceive me." (Descartes 1641: believed in a Catholic God and in the power of the devil.)
After establishing his overwhelming system of doubt, one thing remained clear to Descartes. Even if he was deceived in what he sensed, what he felt, or imagined, or reasoned, one objective certainty held true: He could not be deceived about his existence. If he was using his mind, he was thinking. If he was thinking, he had to exist. Since his Discourse on Method was penned in Latin, thus was born the eternal statement: Cogito, ergo sum (I think, therefore I am).
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