Final answer:
Descartes's next area of exploration after establishing his existence was the distinction between the mind and the body, leading to the development of his dualism theory.
Step-by-step explanation:
After establishing his own existence with the statement "I think, therefore I am" (cogito, ergo sum), René Descartes's next subject of inquiry was the nature of reality and the distinction between the mind and the body. This leads to his theory known as dualism, which posits that there are two distinct types of substance: the mental and the physical. Descartes explored the concept that the mind, or the thinking thing, exists independent of the physical body, suggesting that the essence of the self is non-material. However, his arguments have been met with criticism, as modern science has shown that thought processes are tied to the physical structure and function of the brain. As part of his philosophical exploration, Descartes utilized Cartesian skepticism to cast doubt on sensory experience, considering even the possibility that an evil demon could be deceiving him, as a way to reach a foundation of indisputable knowledge.