The "Introduction" to William Blake's Songs of Innocence tells the story of how the book came to be: a visionary tale that will explore both Blake's rich, complex idea of innocence and the mysteries of creative inspiration.
As the poem begins, the speaker, a piper, is wandering through the "valleys wild" at his leisure, "piping songs of pleasant glee"—enjoying making music all by himself in a green wilderness.
But he isn't alone for long. He's brought up short by a strange encounter:
On a cloud I saw a child.
It's as if his piping has summoned a spirit. The sudden appearance of this hovering, laughing cloud-child doesn't seem to surprise the speaker one bit. He presents this apparition in language pure, plain, and matter-of-fact as a nursery rhyme's—as Blake will present many of the wild visions in Songs of Innocence.
Notice the unfussy structure here: the quatrains (or four-line stanzas) written in bouncy trochaic tetrameter (that is, lines of four trochees, metrical feet with a DUM-da rhythm, as in "Piping | down the | valleys | wild"). Notice the catalexis—the final cut-off unstressed syllable—that makes the lines seem to hang in the air for just a moment when they end, like a kid in mid-skip. Notice, in short, the way the poem's energetic form and simple language feel like they could have come from a picture book. (And indeed they do: Blake beautifully hand-engraved and illuminated almost all of his major works, and no reading of a Blake poem is complete without some time spent pondering the images that converse with the words.)
But already, that simplicity is deceptive. Look carefully at the structure of the first three lines:
Piping down the valleys wild
Piping songs of pleasant glee
On a cloud I saw a child.
Grammatically, the person "piping down the valleys wild" could be either the speaker or the child here. And while the poem will soon resolve that ambiguity—the piper is indeed the speaker—the blurring of boundaries at the beginning is meaningful. In ways that will only become richer and more complicated, the speaker and the child have something to do with each other.
Already, readers can guess that the appearance of a child on a cloud might be a fitting introduction to a collection called Songs of Innocence: children and innocence are pretty much synonymous, symbolically speaking.
Again, though, this is no ordinary child, and the innocence he represents will be a far more complex notion than some sentimental idea of wide-eyed childish wonder. He's here to ask something of the wandering piper—and, "laughing," he speaks.