Besides this, it is a great strain on the nerves because the nerves must be fine in order to get a communication. The intuitive centers in the body are made of fine nerves, finer than one can imagine. They are not matter, they are not spirit; they are between. When once these fine nerves have become sensitive, then the communication is open with the other side. But then the difficulty is that the gross vibrations of this earth are too hard on the nerves and the nerves cannot answer the demands of this gross world, this material world; they become too fine. The result is a nervous illness.