But as I said it, swift there passed me by On noiseless wing a 'wildered butterfly,
Seeking with memories grown dim o'er night Some resting flower of yesterday's delight.
As the speaker is thinking about the nature of all our alone-ness (remember that he's still metaphorically "speaking" in his heart here), a butterfly flutters by.
It's flying quickly and quietly—and maybe a bit haphazardly. It seems to give off a bewildered ("'wildered" is an abbreviation for that word), confused vibe (12).
Apparently this butterfly is looking for something, too—just like our speaker. In this case, the butterfly is after a flower that it remembers having visited the day before. It was probably a great place to chill and do butterfly stuff, like…um, flap your wings and look pretty. We're really not sure what butterflies are into.
Remember, though, that someone has cut everything down. That flower is probably long gone by now. Poor butterfly—no wonder it seems so out of sorts. Where can it chill now?
And once I marked his flight go round and round, As where some flower lay withering on the ground.
And then he flew as far as eye could see, And then on tremulous wing came back to me.
The speaker watches ("I marked") the butterfly flying around and around, looking for the flower.
We hate to break it to the bug, but that flower is no more. It's been cut down by the grass cutter and is just withering there on the ground. Tough break, butterfly.
Then the butterfly flies off, almost out of sight, and comes back again.
As it flies, the speaker describes its wings as "tremulous," or shaky. We get it. If someone chopped down our favorite hangout, we'd be pretty shaky, too.
I thought of questions that have no reply, And would have turned to toss the grass to dry;
But he turned first, and led my eye to look At a tall tuft of flowers beside a brook,
A leaping tongue of bloom the scythe had spared Beside a reedy brook the scythe had bared.
The speaker takes a hard right turn from his observations of the butterfly. He's now thinking of "questions that have no reply" (19). "What's the sound of one hand clapping?" comes to mind. The point here is that the speaker is reflecting on the BIG questions of life. He may be wondering something like, "What does it mean to be all alone in the universe?"
Of course, there's no answer or reply to a question like that, so…let's go back to the butterfly.
The speaker's about to get to work tossing the grass, but just then the butterfly finds a "tall tuft [bunch] of flowers beside a brook" (22)—score.
The little bunch of flowers is described metaphorically as a "leaping tongue of bloom" (23). That's a pretty hopeful description that seems related to the questions that have no reply. Maybe this leaping tongue can communicate some answers.
The grass cutter's scythe has "spared" this bunch of flowers, which lends their existence more weight. It's as though they've been pardoned from an execution. Can things be looking up here?