|A Portrait of Descartes|
Bertrand Russell makes this point in his The History of Western Philosophy:
Descartes's fundamental certainty, "I think, therefore I am," made the basis of knowledge different for each person, since for each the starting-point was his own existence, not that of other individuals or of the community. His emphasis upon the reliability of clear and distinct ideas tended in the same direction, since it is by introspection that we think we discover whether our ideas are clear and distinct. Most philosophy since Descartes has had this intellectually individualistic aspect in a greater or less degree.Descartes was not a philosopher of individualism, but his Cogito led to individualism becoming one of the central concerns in philosophy.
Russell traces the origin of individualistic trends to Ancient Greece, where individualism arose as a reaction to the loss of political liberty in the time of Alexander. While the Platonic and Aristotelian schools in Greece saw man as a member of the community, the Cynics and Stoics were supportive of individualism. The Christian mystics, who were influenced by the Stoics, kept individualistic trends alive in Christian ethics. The coming of Protestantism further strengthened the individualistic tendencies because for the protestants truth is not a social but an individual enterprise.