The speaker of Percy Bysshe Shelley's "To a Skylark" is amazed at the sheer joy of the bird to whom he listens. As the bird's "rain of melody" showers down upon him, the poet wonders how such "a flood of rapture so divine" can issue from this creature who has not known "love's sad satiety." Unlike this bird from whom joy issues without its having experienced pain, the speaker remarks that humans look at both the past and the future, and their happiness is often bittersweet as it is mixed with some past sorrow. In fact, the speaker observes, "Our sweetest songs are those that tell of saddest thought." That is, men's happiness is often measured by the extent to which they have suffered because, for humans, it is not possible to experience deep happiness unless they have truly known sorrow; these emotions are in direct proportion to one another. Perhaps, as Shelley supposes, the happiness of the lark is in its mind--"Teach me half the gladness/That thy brain must know"--while men's is in their hearts and,so, must include all emotions.