Final answer:
Descartes asserts his existence as a thinking entity through his cogito argument, 'I think, therefore I am,' establishing dualism between mind and body where the mind exists independently of the physical world. His foundational belief in his existence as a res cogitans stands despite later scientific objections.
Step-by-step explanation:
In Meditation II of Meditations on First Philosophy, René Descartes arrives at the certainty of his existence as a res cogitans, or a thinking thing, through a foundational philosophical assertion famously summarized as "Cogito, ergo sum" ("I think, therefore I am"). Descartes employs methodical doubt to strip away all beliefs that could possibly be doubted. What remains is the indubitable truth that if he is thinking, then he must exist in some form, even if all sensory experience is an illusion. This thought process leads him to establish a dualism between the mind (res cogitans) and the body (res extensa), positioning the mind as a non-material entity that does not require a physical body to exist. Cartesian skepticism opened the door to this realization by allowing Descartes to question the validity of his own senses, leading him to rely purely on the act of thinking as evidence of his existence.
Though this argument has been intensely criticized and rejected in light of modern neuroscience—which posits that consciousness and thought are inextricably tied to the physical brain—Descartes' proposition remains a central pillar in the history of philosophy and has deeply influenced the mind-body problem and notions of self and consciousness.