A symbol is indeed the only possible expression of some invisible essence, a transparent lamp about a spiritual flame; while allegory is one of many possible representations of an embodied thing, or familiar principle, and belongs to fancy and not to imagination: the one is a revelation, the other an amusement.
Even when the poet seems most himself . . . he is never the bundle of accident and incoherence that sits down to breakfast; he has been reborn as an idea, something intended, complete.
When you are old and gray and full of sleep, and nodding by the fire, take down this book and slowly read, and dream of the soft look your eyes had once, and of their shadows deep.