Blake begins with an accident so small that most of us would forget it within seconds: a hand moves, and a fly is gone. The speaker doesn't forget it. Over twenty lines the brushed-away insect becomes a mirror, and what the speaker sees in it is his own life, just as brief and just as exposed to something that never notices him.
The poem moves in clear steps. The first stanza is a kind of apology: "Thy summer's play / My thoughtless hand / Has brushed away." Calling the fly's life "play" gives it a lightness and a purpose of its own, while "thoughtless" puts the blame on carelessness rather than cruelty. The hinge comes in the second stanza, in two questions that trade places: "Am not I / A fly like thee? / Or art not thou / A man like me?" Each question is the other turned inside out, so neither creature is allowed to be the superior one. The third stanza makes the comparison concrete. The fly's "play" becomes the man's dancing, drinking and singing, and the hand that killed the fly returns as a "blind hand" that will one day "brush my wing." The speaker has quietly given himself a wing, so the swap of bodies is already complete.
The verb matters here. To brush is what we do to crumbs or dust, a gesture too casual to count as violence. By using the same word for the fly's death and his own, the speaker makes human death sound just as trivial. The hand also shifts from "thoughtless" to "blind." It is not malicious, merely unseeing. The poem never says whose hand it is, whether chance, time or something larger, and that silence leaves the reader to decide.
The fourth stanza turns to argument: "If thought is life / And strength and breath / And the want / Of thought is death." The "if" sets it up like a logical proof. "Want" most plainly means "lack," though the modern sense of desire hovers behind it. The conclusion in the final stanza is strange: "Then am I / A happy fly, / If I live, / Or if I die." One reading runs as follows. If to think is to live, then a man who can think this way is alive, and if he ever stops, he dies as the fly did. Either way he is no worse off than the fly, so he can call himself happy. But the line can also be heard as a shrug, or even as bitter irony, and the poem doesn't say which. There is a further twist. The speaker began as the thoughtless one, and the poem itself is the thought that earns him life by his own definition.
The form works against the weight of the subject. The lines are short, usually three or four syllables, and the words are almost all plain one-syllable ones, so the poem bounces along like a nursery rhyme. Rhyme falls on the second and fourth lines of every stanza: play and away, thee and me, sing and wing, breath and death. In the last stanza the sounds tighten. "I," "fly" and "die" all share one vowel, so the speaker, the insect and death are bound into a single sound. The first line already hints at this, since "Fly" is answered by "Am not I" two stanzas on. The light, tune-like movement is itself a small irony: it suits something that dances and drinks and sings, while the subject is how soon that dance can end.
The poem is significant because it gets so much from so little. It starts with a gesture nobody would think twice about and turns it into a question about how much any life weighs, human lives included. It doesn't lecture or announce a moral. It asks the reader to feel the comparison, and then it leaves the ending open, since the happiness of that final "fly" depends entirely on how you read the "if."