In Descartes Meditations he talks about many different things.Two things he talks about are his arguments on Dreams and what he calls the Evil Demon. Both of them have to do with deceiving the mind into believing in things that may be untrue.
The dream argument has to do with that we never truly know if we are dreaming or we are awake. Since there is no way for a person to tell if they are dreaming or not, a person realistically can not tell if anything they have ever experienced or is experiencing is real or just a mere dream. We decide what is real what we can sense; the problem is that your brain can trick your senses. Your dreams can make you feel like you are doing something, seeing something, or feeling any other sense. Now this fits right in with the Evil Demon argument. In the Evil Demon argument you are being tricked by an outside source. This outside source can do absolutely anything, it can control all of your senses and you have no power at all to stop this being. So if the being can control your senses nothing you sense, or believe is there, actually is there.
If either of these things is true have we truly ever done anything? What are we truly doing? Meanwhile I have written this entire paper have I really written anything? For all I know is that I dreamed I writing this entire paper. Yes my senses tell me I typed this paper. I had to think of what to type and meanwhile I was typing the paper I was getting really bored, then my sense of hearing hears the theme song to the TV show The Simpsons. Was this fabulous TV show actually on, or was some superior being just playing with my mind? My feeling is that it really doesnt matter if it was real or I enjoyed the show and the half hour break I believed I had meanwhile I was watching it.