Mirrors — those of the symbolic flavor, i.e. mirrors of the soul — don’t necessarily have to be visual. In one of Neil Gaiman’s short stories, a narrative work of art (i.e. a story-in-a-story) does the same trick that Oscar Wilde’s painting of Dorian Gray performs. Thus Gaiman removes the symbolic mirror one step further from literal mirrors.
Imagine you sit model for a painting. When it is finished, you look at it with surprise and admiration: the painter is a true artist, and this is his masterpiece. It brings out something about you, your true personality; it shows things of which you were only dimly aware yourself. To use an old expression: it reflects your soul.