The poet takes a moment to consider his spontaneous and unthinking action of killing a fly.
It appears in a collection entitled ‘Songs of Innocence and Experience’ published in 1794, and if it seems more than a little mystical in tone we should not be surprised, due to the poet’s regular mystical visions and meditations.
In a moment of introspection and understanding, it occurs to him that the fly is a living creature like himself, and has its own unique existence - a life that, to the fly, might seem equally as important as the poet’s life seems to him.
The poet, realising that he too lives much of his life in a self-centred, creature-like existence (also dances, drinks and sings), concludes that at such times he might be no better or worse than the humble fly. He is equally as vulnerable – unaware of things that might be greater and more powerful than himself.
By considering the commonality of all living things, the implications of even our most insignificant deeds must be recognised (a view expressed in many mystical or religious teachings - as well as being quite a modern idea in terms of environment and conservation).
The poet also reflects that the very process of contemplating this idea, the process of thinking, is a sign of life, and that, on the other hand, death would perhaps be a state without thought and, therefore, without sorrow. He is happy - a happy fly - if he lives or if he dies.