"Expostulation and Reply" begins with an exasperated outburst from an as-yet-unknown speaker. This, readers will soon learn, is Matthew—and he's fed up with his old friend William, who's sitting on an "old grey stone" as if he didn't have a care in the world.
Right away, Matthew sounds like a pretty exacting guy. He notices that William has been sitting there for precisely "half a day," for instance, and feels that to sit thoughtfully alone like this is to "dream your time away." And his voice suggests that he feels rather indignant about the way that William chooses to use his time: he asks "Why William" not once, but twice, his emphatically alliterative repetition suggesting his righteous bafflement. These first words also hint that part of what he objects to is the way William is sitting alone, not participating in the social world.
In leaping straight into this dialogue, the poem bursts in on the reader just as Matthew bursts in on William's peaceful daydreaming. This abrupt, lively beginning introduces what will become a debate between, not just two friends, but two approaches to life: an active, striving mode, and a passive, receptive mode. The question at hand is: does all wisdom come from the active pursuit of knowledge and from the study of what humanity has already discovered? Or are there subtler, quieter, more personal ways of knowing and learning?
That the daydreamer here is named "William" suggests that the author of this poem—one William Wordsworth, don't you know—is likely to think at least a little differently than the idealistic-but-blustery Matthew. This will be a poem about what one can learn from being not a seeker, but a receiver.