What was the theory of Descartes?

He cannot, therefore, propose substance dualism as a theory of mind. To explain mental activities such as sensation, memory, or imagination, one must hypothesize how they result from interaction between the environment, the senses, and the processing of the brain. Descartes initiated such a naturalized account.

Keeping this in view, what is Descartes most famous for?

Descartes has been heralded as the first modern philosopher. He is famous for having made an important connection between geometry and algebra, which allowed for the solving of geometrical problems by way of algebraic equations.

What did Descartes discover?

Descartes' influence in mathematics is equally apparent; the Cartesian coordinate system (see below) was named after him. He is credited as the father of analytical geometry, the bridge between algebra and geometry, used in the discovery of infinitesimal calculus and analysis.