In "The Philosophy of Composition," Poe states that he decided on the
effect he wanted to create in the poem "The Raven"
first. He then worked backward from that desired effect to determine the other elements of the poem, such as the tone, theme, and even the specific words used.
Specifically, Poe says he wanted to create a poem that would evoke a "deep sense of melancholy" in the reader. This desired effect then led him to choose a raven as the central symbol, a dark and ominous bird that would reinforce the poem's melancholic atmosphere.