The last two lines of the octet, though, present a paradox. The speaker notes that the beauty of an object, like a flower, on a summer's day will eventually decline or lose it beauty. But with that statement he is suggesting that the beauty of the young man will not fade or disappear. How can this be? In several of the earlier sonnets the speaker warned the young man that he better have children because his beauty will fade. But now he is saying that his beauty will not fade.
The speaker presents this paradox even more directly in the first line of the sestet:
Eternal does mean forever, for all time; and the word summer here is used comparatively to suggest the beauty of the young man. The next two lines emphasize this same idea. The speaker asserts that the young man will never lose the beauty (the "fair") that he possesses. And Death (personified) will never be able to brag or boast that he has taken the young man.
The paradox is explained at the end of the quatrain (line 12). The beauty of the young man will survive and will live forever in "eternal lines." Lines refer to lines of poetry. The beauty of the young man is captured, like a photograph, in the lines of poetry written by William Shakespeare. The young man's beauty becomes immortal by "Sonnet 18." The word this in the last line of the sestet refers to this