In Raymond Carver's "Cathedral," a reader might find many themes, but there is one that stands out for me.
It's important to remember that a theme is a major idea or "life truth" that the author is trying to share with the reader.
It is interesting to note the kind of man the narrator is. He is resentful of the platonic relationship his wife has with a man she worked with in the past. Having lost his wife, he is traveling; they have planned to have him stay one night before moving on. If the narrator isn't already being difficult about their impending visitor, the fact that Robert is blind does not make things easier. In truth, the narrator acts very much like a child, unhappy about specific aspects of the impending visiting; specifically, he is prejudiced about Robert being blind:
I wasn't enthusiastic about his visit. He was no one I knew. And his being blind bothered me. My idea of blindness came from the movies. In the movies, the blind moved slowly and never laughed.
The speaker makes it clear that he is not happy about Robert coming; without knowing him, the narrator draws unfair conclusions about the man, and his wife makes it clear that he had better behave himself while Robert is visiting them:
"If you love me," she said, "you can do this for me. If you don't love me, okay. But if you had a friend, any friend, and the friend came to visit, I'd make him feel comfortable."
When Robert arrives, things are a little awkward, but perhaps no more than any people meeting for the first time in a social setting. The narrator's wife has pointed out to him that he has no friends. He seems comfortable in his own element, without strangers interfering with his isolation. However, as the night progresses the two men talk and eventually "watch" television, as the narrator tries to describe with words what a cathedral "looks" like. Robert suggests that if the speaker will get a large piece of paper and a pen, while the narrator draws Robert can follow the movement of his hand and mentally envision the shape of the building.
He found my hand, the hand with the pen. He closed his hand over my hand. "Go ahead, bub, draw," he said. "Draw. You'll see. I'll follow along with you. It'll be okay. Just begin now like I'm telling you. You'll see. Draw," the blind man said.
The key to the theme is in Robert's comment: "You'll see." The narrator believes that "seeing" is something he can do that Robert cannot. He thinks that Robert as a man of limitations, but the truth is that the narrator is the blind one: unable to see the value of the world and the people around them because he bases his perceptions on what can be seen. This exercise with Robert opens the narrator's mind to how blind he really is, but the reader is left with the impression that the speaker has been changed by understanding how to see as Robert does (a paradoxical development). The narrator draws with his eyes closed:
"Keep them that way," [Robert] said. He said, "Don't stop now. Draw."
So we kept on with it. His fingers rode my fingers as my hand went over the paper. It was like nothing else in my life up to now.
At the close of the story, the narrator is not interested in seeing with his eyes open, but continues to see the world with his eyes shut. The theme could be that things are not always what they seem, or one cannot know the world simply by looking at it: one must look beyond the surface.