in the system of René Descartes, the knowing subject or ego. The Cartesian self is capable of one fundamental certainty because, even if all else is subject to doubt, one cannot seriously doubt that one is thinking, as to doubt is to think. Thus, Descartes concludes, cogito ergo sum (“I am thinking, therefore I exist”). From this position, Descartes argues that all ideas intuited by the self with the same clarity and distinctness as the cogito must be equally true; this enables the intuition of further indubitable truths, such as the existence of God and the external world. However, since the ideas clearest to the self are the contents of the mind, the notion of the Cartesian self has led to a radical dualism between the inner life of the mind (subjectivity) and the outer world of things (objectivity). It has also led to the idea that knowledge is necessarily subjective and has opened the question as to how the outer
world, including other human beings, can be known except as an idea. See Cartesian dualism; egocentric predicament; solipsism.