"Cathedral" looks at the unstable nature of reality. For example, the unnamed narrator's reality, or his vision of Robert, a blind man, is based on images from the movies about blind people, his wife's descriptions, and Robert's voice on tape. When the narrator meets Robert for the first time, the reality of Robert, for the narrator, changes. This reality continues to change throughout the night as they get to know each other better. The story's ending suggests that by the end of the night, the narrator has learned that nothing was quite as he thought. His experience with Robert, and attempting to experience the world from Robert's perspective, opens up a whole new way of looking at the world. This is complicated by the fact that narrator's big experience happens when he is under the influences of alcohol and marijuana. Does this change the experience? This is just one of the questions provoked by Raymond Carver's most famous story.
Robert gains a more positive version of reality as a result of his encounter with the narrator.
"Cathedral" stresses that assumptions hinder our ability to accurately perceive reality.