One can see pictures of Buddha, with closed eyes, sitting cross-legged. What does that symbol convey to us? That there is a knowledge that can be learnt by closing not only the eyes but also the mind from the outside world. Closing the eyes does not make the concentration any greater. Most people go as far as closing the eyes, and no further; but if the eyes are closed and the mind is pondering over things, that is not concentration. Those who can concentrate can do it without having to close the eyes.