Foreseeing the young man's inevitable decline into age and infirmity, the speaker fortifies his memory against time's cruelty, vowing that his verses will preserve the beloved's beauty eternal even as the flesh withers.
Sonnet 63 is unusual in the sequence for its prophetic stance. The speaker is not remembering what has been lost but forecasting what will be lost. Lines 1–8 construct a vivid future landscape of decay: drained blood, wrinkled brows, the theft of spring. This is uncanny—the speaker speaks as though aging has already happened ('shall be as I am now'). The effect is hallucinatory: the young man's future age becomes present through language. Yet this is precisely the speaker's strategy: by making decay imaginatively real now, he prepares his defenses. The fortification he undertakes (line 9) is psychological and literary—a preemptive strike against time through commemorative verse.
The couplet offers the classical response to mortality: 'His beauty shall in these black lines be seen.' The 'black lines'—literal ink on paper—become the immortal vehicle for the beloved's eternal youth. Yet Shakespeare's formulation is careful: the lines will live; the beloved will 'in them still green'—he exists not independently but within the poetry. This is both redemptive and troubling: the price of immortality is absorption into art. The young man's body may age and die, but his essence lives perpetually in the poem's 'black lines,' becoming a kind of ghostly immortality. The poem preserves not life but its image, not the person but the beloved as fixed in time.
Like taking photos of someone you love because you know they'll change, or writing down memories before they fade. The speaker is saying: 'I can't stop you from aging, but I can make sure the world never forgets how you were beautiful.'