Final answer:
In Shakespeare's 'Sonnet 18,' the line in question implies that the beauty of the beloved is eternal and unchanging, and will live on through the enduring nature of the written word.
Step-by-step explanation:
The line from Shakespeare's Sonnet 18, "nor lose possession of that fair thou ow'st," suggests that a central idea of this poem is that the beloved's beauty is eternal and unchanging. Shakespeare uses vivid imagery to compare the beloved to a summer's day, but notes that unlike a summer's day, which is subject to the changing seasons and can be marred by winds or fading beauty, the beloved's beauty will be everlasting.
This is accomplished through the power of the poet's verse, which will capture the beloved's beauty and make it immortal. As supported by references from Brooks's reading of Keats's "Ode on a Grecian Urn" and LibreTexts, poetry and art possess the ability to eternalize beauty and preserve it from the ravages of time.