Another very important medium of influencing others, including our old Active partner, is the Eye. The Human Eye! Who does not know its power, and yet how few know how to acquire the secret of its use! One could write volumes upon its power as a weapon of offense and defense, and its aid in influencing man and the lower animals, and yet not begin to exhaust the subject.
Does "Who does not know its power" imply "many people know its power"?