Donne begins with a walk and a flower and ends up doing sums. That slide from meadow to mathematics is the poem's joke and its argument. The speaker climbs Primrose hill hoping to find "a true Love," and the sight of the flowers sets off a chain of reasoning that grows more outrageous at every step.
The opening stanza builds a grand picture. If heaven would "distil / A shower of rain," each drop would find "his own primrose" and turn to "manna," the food the Bible says fell from heaven, so an ordinary hillside becomes holy ground. The flowers are so many that their "infinity" makes "a terrestrial Galaxy," the earth's own copy of the stars. All of this runs on in one breathless sentence before the speaker reveals why he is here: "I walk to find a true Love." The cosmic build-up then narrows into a smug deduction. She cannot be "a mere woman," so she "must or more or less than woman be."
From here the primrose becomes a measuring instrument. A primrose has five petals, and the speaker wonders whether he should wish for "a six, or four." Petals stand for degrees of womanhood, and both extremes fail. A woman who is less than woman would be "scarce any thing." One who is more would "get above / All thought of sex," and the line break lifts her out of reach, so that he would "study her, and not … love." Both are "monsters." Then comes the poem's most uncomfortable moment. Since "there must reside / Falsehood in woman," he would rather she were falsified "by art than Nature." The joke rests on an assumption that women are false, and the poem invites us to ask whether Donne shares the speaker's smugness or is putting it on display.
The form helps the performance. Each ten-line stanza opens with two short six-syllable lines ("Upon this Primrose hill, / Where, if Heav'n would distil") before stretching into longer, argumentative lines. That makes each stanza feel like a quick breath followed by a speech. The rhymes also speed up. The first stanza begins with tidy couplets (hill/distil, go/so), then a run of long "ee" sounds (infinity, Galaxy, see, she, be) piles up, as if the speaker's confidence were snowballing. In the third stanza the final three lines close on "all / fall / all," and the repeated word gives the last line the snap of a punchline.
That last stanza is where the logic turns slippery. The primrose is told to "thrive / With thy true number five," and woman is told to be "content" with this "mysterious number." Five is half of ten, "the farthest number," so each woman gets half of ten and "may take half us men." If that is not enough, the speaker reaches for a second argument about odd and even numbers, and the reasoning is deliberately hard to pin down. The words "if," "since" and "then" give it the look of a scholarly proof, yet it ends in a pun. "Take" has a sexual edge, and the final claim, "woman may take us all," reverses the poem. The man who set out to choose a true love finds that he is the one being chosen, and not just once.
The poem is significant because it shows how Donne's love poetry can turn feeling into argument and argument into comedy. In a few lines it mixes the sacred (manna), the astronomical (a galaxy) and the numerical (petals and sums), and the incongruity is the point. It also deflates romantic idealism. The true love turns out to be neither goddess nor nothing but an ordinary woman, five petals and all. Whether that conclusion is praise, condescension or a bit of both is for the reader to decide, and the poem's cleverness leaves that question open.