Owen is writing about the central ideas in "Sonnet 18” by William Shakespeare. But thy eternal summer shall not fade, Nor lose possession of that fair thou ow'st, Nor shall death brag thou wander'st in his shade, When in eternal lines to time thou grow'st, So long as men can breathe or eyes can see, So long lives this, and this gives life to thee. What is the central idea of these lines?