The poet questions what his mind can possibly contain or communicate that his speech has not already exhausted, suggesting an endless cycle of repetition and inadequacy.
To engrave or write—but also to define or delineate. Ink can inscribe what the brain already contains, but cannot add or create anything new.
Sonnet 108 treats language as a closed, finite system: speech has already characterized the beloved fully and exhaustively, and the poet can only repeat what has been said. The brain as receptacle for thought, ink as mere transfer of pre-existing mental content. If all possible praise has already been articulated, both brain and ink become redundant. The sonnet expresses profound anxiety about the futility of any artistic act that follows earlier adequate expression.
The sonnet's resolution is subtle and consoling: repetition is actually fidelity. If the poet says the same thing eternally, it is because the same truth is eternally unchanged and unchangeable. Constancy and repetition become synonymous—the poet's inability to articulate anything new is a sign of unchanging love. The poems derive value not from novelty but from their loyal reiteration of a truth so stable that only endless repetition can adequately honor it.
Trying to explain why you love someone after you've already explained it a hundred times—running out of new ways to say the same thing, knowing each repetition feels weaker than the last.