Four short lines, and the speaker has already picked a quiet fight with certainty. The poem opens with a confession: "An ill fit for the church / of mathematics am I." Nobody is accused of anything. The misfit is the speaker, and the tone is closer to a shrug than a protest. That modesty matters, because the poem never says that mathematics is wrong. It says only that its certainties are a place where the speaker cannot sit comfortably.
The metaphor in the first sentence carries most of the weight. Mathematics is the discipline we associate with proof, the one subject where "I think so" is not good enough. Calling it a "church" recasts it as a matter of faith, with a congregation, a doctrine and a place where one either belongs or doesn't. The irony is neat: the realm of reason is described in the vocabulary of devotion. "Fit" works in two directions as well. It suggests a garment that pinches, or a pew that was built for someone else's frame, and so it makes the speaker's unease physical rather than merely intellectual.
The word order adds to this. "Am I" is placed last, in an inverted construction that sounds old-fashioned and liturgical, like a line from a prayer book or a creed. The speaker uses the solemn music of belief to announce their unbelief. The poem has no regular rhyme, but it is carefully balanced. The lines run to about six, seven, seven and six syllables, so the poem swells slightly in the middle and tapers at the end, as if it were settling toward sleep. The line endings also chime softly. "I," "doubt" and "night" all end on a gliding vowel, and "night" in particular answers "I" from two lines earlier. The speaker and the darkness are linked at the level of sound before the poem says anything about them.
The second sentence turns from confession to challenge: "Show me what is lost by doubt." "Show me" is the language of the sceptic, and of the mathematician too, who also wants to see the working. The speaker borrows the demand for evidence and aims it at the believers in certainty. If doubt is supposed to cost us something, let them prove it. The sentence works as a dare, and the poem leaves the possible answer unspoken. The silence suggests either that nothing can be shown or that whatever is lost is a price the speaker is willing to pay.
The last line complicates things pleasantly: "or a long-desired night." Grammatically, it is unclear whether "night" is a second thing that might cause a loss ("lost by doubt or a long-desired night") or something the speaker wants to be shown. Both readings are possible, and the poem does not choose between them. That ambiguity suits a poem called "Against certainty", since a line that could be settled with one correct parse would undercut the argument. The phrase "long-desired" also changes what night means. Darkness is usually the thing proof is meant to rescue us from, a symbol of ignorance and confusion. Here it is something wanted, and wanted for a long time. Desire is an odd word to set beside mathematics, because it belongs to the body and the heart rather than to the proof. Night can be rest, or intimacy, or simply a mind allowed to stop settling things. The poem does not specify, and it ends there, without a resolving sum.
What makes the poem worth reading is how much it does without raising its voice. It does not argue against certainty so much as demonstrate a temperament that prefers the unlit hours. By ending on "night" rather than a conclusion, the poem lets its form agree with its claim: it trails into the dark instead of closing with a neat QED. For a school reader, it is a useful example of how a very small poem can hold a whole philosophical position. A single extended metaphor, one challenge and one ambiguous image are enough to make doubt sound less like a failing and more like a place one might choose to go.