"If you see my love aging, work against it." is the central idea of the third quatrain.
Shakespeare's Fair Youth Sequence includes 100 sonnets, one of which is Sonnet 100.
The poet's passion for a young man is discussed in the sonnet. In the sonnet, the poet addresses the muse and inquires as to her whereabouts. Has she forgotten to motivate the poet? The poet begs the muse to come back to him and make up for the lost time. In order to write about his love before the lover ages, he seeks the muse for assistance. The poet sought the muse's assistance in order to immortalize his love for the beloved.
Therefore, the central idea of the third quatrain is "If you see my love aging, work against it."
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