All knowledge is built on knowledge of myself, which I must know for myself. Cultural differences aren't highly valued. Each of us, according to Descartes, should decide about our culture for ourselves. Human being is a very social creature. Aristotle thought this was obvious from experience.
Human beings are social creatures who depend on others to be what they are.
The social side to ourselves is very important. We need status, acknowledgement. We need each other because we care about what others think about us. We see ourselves through the eyes of others. For Descartes, reasoning, western human being is the model for every man.