Just as our physical eyes see things in the physical world, so the inner eyes sees things and images that belong to other worlds and subtle images of things of this physical world also. Further, vision is of value because it is often a first key to inner planes of one's own being and one's own consciousness as distinguished from worlds or planes of the cosmic consciousness. Yoga-experience often begins with some opening of the third eye in the forehead (the centre of vision in the brows) or with some kind of beginning and extension of subtle seeing which may seem unimportant at first but is the vestibule to deeper experience. Even when it is not that, for one can go to experience direct, it can come in afterwards as a powerful aid to experience; it can be full of indications which help to self-knowledge or knowledge of things or knowledge of people; it can be veridical and lead to prevision, premonition and other openings of less importance but very useful to a yogi. In short, vision is a great instrument though not absolutely indispensable.