Beyond question Descartes was the chief architect of the 17th century intellectual revolution, which destabilised medieval scholasticism and laid down the philosophical foundations of what we think of as the modern scientific age.
As a small boy Descartes was sent to the newly founded collage of Le Flèche in Anjou (France) where he received from the Jesuits a firm grounding in the very scholastic philosophy he was subsequently to challenge. In his early adulthood Descartes came to see in the methods and reasoning of mathematics the kind of precision and certainty which traditional philosophy lacked. Much of Descartes early work was what we should now call scientific. A corner stone of his approach was that the matter throughout the universe was of essentially the same type. Hence the earth was merely part of a homogeneous universe obeying universal physical laws. At this time in the mid 17th century such views could be dangerous and he withdrew his work from publication in 1633 on hearing of the condemnation of Galileo by the Roman inquisition for advocating that the earth revolved around the sun (which Descartes believed). But in 1637 he released anonymously a sample of his work called: "Geometry Optics & Meteorology" prefaced this was what was to become an acknowledged philosophical classic, the "Discourse on the Method of Rightly Conducting Reason and Reaching the Truth in the Sciences" (known as the "Discourse"). The books fame rests on the short central section in which Descartes discusses the foundations of knowledge, the existence of God, and the destination between mind and body. These thoughts together with his philosophical master piece "The Meditations of First Philosophy" published later in 1641 constitute the philosophical core of the Cartesian system.Descartes was particularly concerned with the validation of knowledge, his main aim was to show how the world of physics could be reliably made out independently of the sometimes misleading information provided by our senses. Thus his first meditation is that: "The senses deceive from time to time." Here Descartes introduces his famous "Dreaming Argument". "I think I am standing in front of the fire but it could turn out to be false (I might be asleep in bed)." From this he went on to more radical doubts, perhaps all external things are merely "the delusions of dreams".
This was the first meditation.
However Descartes goes on to say one thing is certain "Let the demon
deceive as much as he may … I am, I exist, is certain, so long as it
is put forward by me, or conceived in my mind".
This is the third meditation.
Descartes reasons that the idea of infinite perfection that he finds within himself is so great that he could not have conceived it from the resources of his own mind.
The cause of such an idea containing such perfection must in itself be perfect and hence must have been placed in his mind by an actually existing perfect being; i.e. God, who guarantees the reliability of human thought. We make errors because we agree to propositions that are not clear. If we use our God given power of reason correctly we can be sure of avoiding error.
This is the fourth meditation.
Each of these conclusions has been subject to enormous discussion. Probably nothing about Descartes has been discussed as intensely as Cartesian dualism, namely the view that the mind or soul (Descartes does not distinguish between them) is entirely distinct from the body and could not fail to be even if the body did not exist. However it is difficult to see how two such incompatibility substances such as mind and body can be viewed in this way. Certainly today there are many who would accept the idea of a thinking self (I think therefore I am) without accepting the mind / body dualism.