Poem found here: "The Fly" by William Blake
From condescending to humble, the poem plays with tone as the speaker plays with the concept of "the fly." The initial stanza, "Little fly, / Thy summer's play / My thoughtless hand / has brushed away" has a sense of play from it. It's dismissive of the existence of the fly as a nuisance. As the poem progresses, there's a change of thought.
"Am not I / A fly like thee?" I wonder what caused this kind of crisis. A twist of the hand? Something as simple as that to go introspective. Well, this type of rash thinking plays with the adjective "thoughtless" as thought the insight is still in play.
"Or art not thou / A man like me?" So a dual simile is set up. The question is what criteria is a man like a fly or a fly like a man: "For I dance / And drink and sing, / Till some blind hand / Shall brush my wing." These lines focus on the physical at first -- singing, dancing, drinking -- and than the comparative hand brushing away the speaker's wing parallels the speaker brushing the fly away. This comparison is a bit trite, but the triteness sobers up the speaker in essence.
It is not the deep thoughts that sober a person up, but that slap of a line or idea.
"If thought is life / And strength and breath, / And the want of thought is death." What triggers from the slap is the thought of death. An overreach. Perhaps, but note that the speaker's feelings fluctuate and zoom in and out at a fast pace. In a short poem like this the forced urgency tells more about the speaker than the thoughts of the speaker. There's a force to keep switching to survive -- size, thought, questioning.
The answer is quite humorous, "Then am I / A happy fly," Is the fly happier knowing it's short but physically fun existence -- why not? Why not just be happy knowing "If I live, / Or if I die."
But the poem ends there -- without an answer but a definite question. Knowing life and death, does that matter and to what scale. And what does it matter to a man or a fly who are both one in the same.