Cogito Ergo Sum - Je ponce donc je suis - I think therefore I am.
A quiet room and a rational mind all is all one may need according to Descartes. If you ask Parisians to define their thought, many of them will tell you they are Descartians. In essence, rationalist, they use their experiences and thoughts to "know". It is an independent method to determine a vision of existence.
Descarte is the father of modern philosophy. But his ideas were far from the last thoughts as later Nietzsche, Camus, Foucault, Satre and others would throw the French people and the world off the tracks of good thinking.
The problem with rationalism is it starts with the self and ends with the self. All of us know life does not work this way. You can create your own vision of existence, but that doesn't mean your choice are true or good.
In his work, "Passions of the Soul" he stated he was starting a fresh with no considerations of the thinkers that came before him. However, his work is full of concepts long dealt with by the Stoics and Christians.
With all his rational philosophy, Descartes was yet Christian. The problem that now exist with his philosophy is can now be explained in a physical reality. Descartes body is buried in a Catholic Church near the latin quarter of Paris, but his head is in The University of Paris where it is still studied today. If you separate the body from the head you do not get the whole picture. Separate faith from thought, and ideas will fail you.
The problem with Descartes can be best summoned up in the lyrics of Rich Mullins song, "Maker of Noses":
"They said boy you just follow your heart
But my heart just led me into my chest
They said follow your nose
But the direction changed every time I went and turned my head
And they said boy you just follow your dreams
But my dreams were only misty notions
But the Father of hearts and the Maker of noses
And the Giver of dreams He's the one I have chosen
And I will follow Him"
If a man starts and ends with himself, that is all he is left with in the end. This is poor thinking.