Jorn K. Bramann, PhD in The Educating Rita Workbook is the source reference for the Descartes: The Solitary Self essay. This excellent treatise deserves your full attention.
"There are two cultural legacies of lasting importance that Descartes’ radical separation of the mind from the physical world has left—two philosophical conceptions of reality that found expression in how Europeans related to their environment, and how they perceived their over-all existence in the world.
The one legacy fastens on the absolute sovereignty of the mind vis-à-vis everything that is not mind. While the external world, including the thinker's body, is subject to the laws of physics and other external contingencies, the mind is not. I, being pure mind, enjoy a supreme degree of independence from my body and everything physical.
The radical separation of mind and body--and of the mental and the physical in general--is known as "Cartesian Dualism." And by attributing to the mind something like sovereignty over the external physical world, it has prepared the way for a distinctly modern conception and experience of reality, a conception which replaced older ways of seeing the world in drastic ways.
The other important legacy that originates with Descartes’ radical separation of the mind from everything physical is the inherently solipsistic individualism that time and again emerged in the course of modern European philosophy. Solipsism is the extremist philosophical theory that I am the only being that exists. This theory is invariably perceived as either comical or crazy by anyone who discusses it, and most philosophers have assumed that there are convincing reasons for dismissing it without much ado. The way Descartes sets up and explains his procedure of radical doubt, however, makes it impossible to avoid the conclusion that the doubting self may indeed be the only being that exists. In spite of all efforts to refute it, Cartesianism remains haunted by the ghost of Solipsism . . . The decisive point of Cartesian doubt is the contention that I cannot go outside of myself, as it were, to check whether what I see is real or not. I am always and irremediably inside my mind, and that always keeps alive the theoretical possibility of the truth of Solipsism."