In the last line of the first stanza of "Dover Beach," Arnold refers to the "eternal note of sadness." This note is a sound made by the sea, but, despite the melancholy tone of the poem at this point, it is not clear what causes the sadness. The speaker goes on to say that this note of sadness is not only eternal but universal:
Sophocles long ago
Heard it on the Ægean, and it brought
Into his mind the turbid ebb and flow
Of human misery.
At the end of the poem, the speaker goes on to equate the sadness he feels with the decline of the Christian faith. This, however, is somewhat more specific than the sound Sophocles heard before the Christian faith began. The cause of the sadness in line 14 is what the philosopher and classical scholar Miguel de Unamuno called "the tragic sense of life." This is the idea that, in the absence of religious faith to redeem and make sense of it, life is inevitably tragic, meaningless, and futile. Arnold expresses this nihilistic view as one which is universal and eternal but which arose with particular force in nineteenth-century Europe because faith in Christianity was collapsing at the time. "Dover Beach" is one of the leading expressions of this idea of philosophical nihilism, which can be found in nineteenth-century poets and philosophers from Tennyson to Nietzsche.