The speaker opens on the defensive. He begs not to be judged "light", as though the charge has already been made, or soon will be. He admits the word has some truth in it ("tho' light at times I seem") and says he bears "the stress of fortune" lightly, with the "innumerable flaws of changeful care". But he insists that seeming light is not the same as being worthless. The rest of the poem argues this through one extended image. Life is a sea voyage, success is a safe arrival in port, and there is more than one way to get there.
The word "light" does most of the work. It first sounds like an accusation, meaning frivolous or unserious. Line 4 then repeats line 1 almost word for word ("Nor judge me light for this"), so the plea comes round like a refrain. By line 12 the word has changed sides. The ships that "pass in before" are carried by "some light breeze", and now lightness is what moves them. The speaker never argues the point outright. He lets the word's meaning drift from a fault to a kind of buoyancy. The pilot is "easy Taste", and "easy" echoes "light" again.
The sonnet's construction suits an argument that is held back and then released. The first eight lines are a single suspended sentence. The speaker keeps interrupting himself with qualifications, and a parenthesis in the middle ("Office forbid to mortals, kept supreme / And separate the prerogative of God!") says that passing judgment belongs to God alone. The verb we are waiting for ("deem") arrives early, but its object, "That seaman idle who is borne abroad", is held back and inverted so the sentence bends before it lands. The rhyme tightens the effect. In the octave the same sound comes back four times, in "seem", "deem", "supreme" and "stream", so the opening feels like a worry that keeps returning. The sestet turns at line 9 with "Not he alone", and the rhyme pattern loosens into an open, alternating rhythm. The lines are regular iambic pentameter, steady enough that the argument never sounds rattled.
The sea imagery is built from contrasts between effort and ease. One sailor "to contrarious seas / Opposes, all night long, the unwearied oar". The verbs are active, the night is long, and the oar never rests. The other sailor "is borne" by a "favouring stream", a passive phrase that shows him doing nothing. The poem does not mock the rower. His success is "high" and "endeared", and he earns respect. What it refuses is the assumption that he is the only one who gets home. The other voyagers sail "with upright keels", which suggests balance and perhaps a little moral steadiness, and they are steered by "easy Taste, the golden pilot". Taste is a good choice of guide. It means instinct and discernment, something one has more than something one earns. "Golden" gives it value without making it laborious. There is a quiet link in the rhymes too. "Endeared" at the end of the toiler's line and "steered" at the end of the poem rhyme with each other, so both kinds of sailor finish on the same sound.
The poem is also modest about itself. The speaker never says he is one of the lucky ones. "Shall they... pass in before" speaks of a general hope, and the first half has already said that no mortal is entitled to rank sailors. This keeps the poem from sounding smug. It asks for a withheld verdict, not for praise. Readers who know Stevenson mainly as a writer of adventure stories, *Treasure Island* among them, may notice how naturally he reaches for the sea here, though the poem itself gives no further clues about his life.
What makes the sonnet worth reading is that it turns a defence of temperament into something close to an argument about fairness. Most of us are quick to read hard work as virtue and ease as laziness. The poem suggests that appearances mislead, since a person who looks as if they are drifting may be on a course that taste and nature have set well. It also manages this in a form that is itself disciplined, with fourteen carefully rhymed lines presenting a case for lightness. The tension between those two things is part of its charm.