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).
This fact was undeniable. But herein lay a monumental problem for Descartes. Having separated mind from body, leaping the certainty gap from one to the other became a serious problem. His simple solution, while acceptable to Descartes, was not readily accepted in philosophical circles. In Discourse on Method, he explains that since God exists, the Almighty could not deceive us about reality or he would not be all-perfect.
Using Rene Descartes reasoning, eliminating the mind-body gap he created has only been partially successful down through the centuries. Other philosophers have simply ignored it, or denied it, or explained it, with their own ideas about how knowledge reaches our minds from the outside world.
My Solution to Cross the Gap
In my mind, however, there is no problem crossing the gap of Decartes’ mind-body dualism. My solution is extremely simple and probably falls more into the realm of common sense than any philosophical answer. 
Above, I briefly talked about Mindy Dog and her instinct to eat when she was hungry. I talked about the simple creature, planaria using its pharynx tube to ingest food. Finally, I mentioned the single celled, slipper shaped paramecium which spends its life hunting for bacterial food.
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