According to Descartes, knowledge of the external world can only be achieved through reason and intuition, rather than through sensory information. He believed that the senses could be deceived and that sensory information is often unreliable, so he sought a more secure foundation for knowledge.
Descartes argued that the only way to achieve certainty and knowledge of the external world is through the use of reason and intuition. He believed that the mind can gain knowledge of truth through clear and distinct ideas, which are not subject to doubt or deception.
In his famous quote "Cogito, ergo sum" ("I think, therefore I am"), Descartes claimed that the only thing he could be certain of was the existence of his own mind. From this foundation of certainty, he sought to build a system of knowledge about the external world through reason and intuition.
Overall, Descartes believed that knowledge of the external world was possible, but it could only be achieved through reason and intuition, rather than through sensory information.