Descartes' dualistic concept of human nature looks at two different aspects of all humankind: our mind and our body. These aspects are the exact opposite of one another. Our mind allows us to think the thoughts we have every day, and our body allows us to do the physical things within our day to day lives.
Descartes came to this dualistic concept by using his theory of methodological doubt. The theory involved thinking about anything and everything he was ever taught. By thinking these things, he came to realize that many things he was told have been false, so he went back to the beginning. He did this by doubting everything he ever thought was true, to find at least one thing he could not doubt.
When finally reaching that one truth, he was able to use it to be the foundation when furthering his search to find more things he could not possibly doubt. The first truth he was able to come to was something he referred to as "cogito ergo sum"(I think therefore I am). The cogito basically states that we are existing to the point in which we can think; however, when looking at the body (physical substance) we cannot be entirely certain that this is real in the sense that the physical things around us can all be imagined with the use of our minds.
The thinking aspect of our dualistic selves can be making us believe the things we see or feel; therefore, we can doubt the physical. At the same, time we know we are thinking, so we cannot doubt the mind. This leads to Descartes' concept of how these two aspects are totally different.
When looking at the physical and mental substances, one can see where Descartes is coming from. The mind can play tricks on us and make us believe that something is real and/or there when it really is not. There can be some sort of "puppet master" making us believe the physical things around us are real; however, when looking at the mind, we can argue that the mind, itself, is made up...