Describe in one hundred words what descartes meant when he wrote: "i think, therefore i am."

When there’s nothing left he still is left with himself and nothing else. Regardless of whether or not he is being deceived by some demon or his beliefs are wrong, he is able to see that even if he has the ability to doubt something he must be existing to even doubt it in the first place. The fact that he can think is what assures himself of his own existence, and a deceiving god cannot negate that. From this point on, Descartes can continue in his examination of reality without worry that he is by all means existing.