Why are they separate, and are they necessarily so?
When I think about martial arts globally, I really wonder about a few things.
Why is this internal stuff limited so strongly and for so long to one culture? Why so few, why so supposedly secret, why not well supported by science? Why didn't someone in Wales or The Northwest Territories or Brazil or Mali find and develop the same thing?
To build further upon what Phi Truong's post.
Your question can open HUGE diverging lines of discussion and analysis, such as theories put forth in Guns, Germs and Steel
and the evolution of society. Philosophical differences that developed between the Occident vs Orient - i.e. Cartesian Duality that developed in the West versus the philosophies of looking inward
such as Buddhism, Taosim, Yoga's etc.