Coleridge builds almost the whole poem out of a single "if". Everything from "If dead, we cease to be" down to the closing lines hangs on a hypothesis: suppose death is total, and nothing survives it. The poem then follows that idea to its conclusions, and they are grim. A human being would be a creature with no purpose, no foundation and no reason to feel anything. Because the whole argument is conditional, the poem reads less as a confession of despair than as a reductio ad absurdum, a way of showing how unlivable an idea is by pushing it as far as it will go.
The first image sets the tone. If life is only "life's brief flash", then we are like "summer-gusts, of sudden birth and doom", things whose "sound and motion" are not signs of anything else but "their whole of being". A gust of wind has no hidden meaning. It is only the noise it makes while it lasts. The next lines turn on a neat pair of nouns: breath, Coleridge says, might be mistaken for life itself when it is really "its task and tent", a job to be done and a temporary shelter. The mention of "a soul like Milton's" raises the stakes, since if even the greatest mind we can think of ends in nothing, no one is spared.
Then the poem turns on humanity itself, and its language becomes more violent and inventive. Man is a "vessel purposeless, unmeant", which is a contradiction before the sentence has even finished, because a vessel exists to hold something. He is also a "drone-hive", a hive buzzing with "phantom purposes", though drones are the bees that make no honey. The sharpest image is the vase. Nature, stepping back to inspect a "nigh-finished vase", absent-mindedly shapes humanity "with restless hands unconsciously". The picture suggests a stray scrap of clay, a "surplus" made without attention or intention. "Blank accident! nothing's anomaly!" compresses the idea into five words: we are not even an exception to anything.
From there the vocabulary shifts to scales and money. We are told to "weigh thy dreams" against hopes and fears as "counter-weights", and laughter and tears are said to "create / And to repay the other", like a debt passed back and forth with nothing gained. Emotions are "costless shadows", and the poem keeps asking, almost mockingly, what the point of them could be: "Why rejoices / Thy heart with hollow joy for hollow good?" The mourner's "hood" and the "lamenting voices" make grief look like a costume. The regress of "Image of Image, Ghost of Ghostly Elf" shows a self thinned out with each repetition, until nothing solid is left to be the original.
The form follows the same collapse. The lines are mostly iambic pentameter, and the opening quatrains rhyme neatly (gloom/doom, fare/declare). As the argument loses its footing, the rhyme scheme loosens and the sentences are broken up by outbursts like "O Man!" and "Blank accident!". The poem also ends on an odd number of lines, with no rounded closing couplet. Instead come the rapid commands "Be sad! be glad! be neither! seek, or shun!", which cancel one another out. The last line, "Thy being's being is contradiction", is a loop in itself, with "being" folded back on its own possessive, and its rhyme with the earlier "shun" and "none" is only a faint echo, as though the poem cannot quite click shut.
What makes the poem significant is the gap between what it says and how it says it. Its argument is that human life, without immortality, would be empty and contradictory. Yet the writing is anything but empty. The drone-hive, the potter's careless hands and the weighing scales are the work of a mind full of purpose, and the questions it asks only matter to someone who cares about the answers. Without stating it, the poem suggests that we cannot really live inside the idea it describes. It is a philosophical position tested as a piece of drama, where the speaker's own energy quietly argues against the hopelessness he describes.