Sidney's speaker opens by admitting that he used to be a fraud, though a harmless one. Before Stella, he says, he was "in nature apt to like": quick to be drawn to a pretty face and quick to call the feeling love. The poem comes from the sequence *Astrophil and Stella*, and this sonnet works as a small confession. He mistook an easy attraction for the real thing, and only meeting Stella taught him the difference.
The imagery of the first quatrain shows how shallow that early feeling was. The beauties he admired were "of many carats fine", so the women are measured like gold or jewels, valued and weighed from outside. His response is hot but empty. His "boiling sprites" (spirits) rush toward them, and he decides he is "full of thee", meaning Love itself. The word "thought" is the giveaway, since he is reasoning his way into a feeling and not undergoing one.
The second quatrain turns this false confidence into mild arrogance. He notices that he lacks the "restless flames" that other lovers say make their souls "pine", and he concludes that they are exaggerating. They are "babes" who "whine" over "some pin's hurt". The contrast is neat, because a pinprick is tiny, sharp and almost comic, while the lovers' complaints are the grand agonies of the Petrarchan tradition. The last line of the octave, "By my love judging what love's pain might be", is the poem's quiet joke. He is measuring everyone else's suffering against his own experience, which was never real love.
At line 9, the sonnet turns on "But while I thus with this young lion played". The change of tone is sudden. The "young lion" is most likely Love itself, a cub that looks like a pet but will grow into something that can kill. The line is a warning, and it lets us see how foolish his confidence was. The next line is a hesitation: "Mine eyes (shall I say curst or blest?) beheld". The brackets make the speaker pause mid-sentence, and the question stays unanswered because Stella has turned out to be both a curse and a blessing. Then comes the name, almost offhand: "Stella; now she is nam'd, need more be said?" He treats the name as if it explains everything. It also means "star", which suits a speaker whose own name, Astrophil, suggests a lover of stars.
Sidney handles form with care here. The octave rhymes *abba abba*, in the Italian pattern. The sestet runs *cdcdee*, so the poem ends on the closing couplet of *so* and *know*, as an English sonnet would. The ten-syllable lines are mostly regular iambic pentameter, which makes the final rhyme feel settled and conclusive.
The closing images change from jewels and flames to schooling. Stella's sight gives him "a lesson new" that he has "spell'd", and he has "learn'd Love right". Spelling is what children do when they are beginning to read, so the proud man who laughed at "babes" is now a pupil himself. The last line sums up the lesson with a paradox: he knows love "as who by being poisoned doth poison know". Only someone who has been poisoned really understands poison, and no description can teach it. The line also quietly admits that the lovers he mocked were right all along.
What makes the sonnet matter is that the speaker is allowed to look foolish. Love poetry of this kind often cast the lover as a noble sufferer, but this speaker begins by ridiculing suffering and has to eat his words. The poem is witty and self-mocking, yet the ending is serious, because a lesson learned from pain cannot be taken back. The sonnet argues, in its own small space, that some kinds of knowledge are only available to those who have been hurt by them.