He says that his mind could bring him only fleeting happiness. This is because of the unsteady nature of human mind. The mind is the most delicate and ficklest thing in the world. Alexander Selkirk could see the images of his native land through his mind's eye. But such pictures were quite unsteady. The pictures came to his mind like slides of photographs only to pass away quickly. Besides such recollections soon turned him back to his sadness.