What characteristics of mankind does Shakespeare think are immortal in Sonnet 18? The last two lines say poetry is eternal; mankind is immortal.

I suppose there is a problem in the understanding of this poem (sonnet 18 by William Shakespeare) on the part of the question-maker. The poet wants certain qualities of mankind to be eternal but he does not say that they are eternal. These qualities are youth, beauty, temperance and so on. Unfortunately, the flux of nature spares none, not even the beauty of the fair friend--"Every fair from fair sometimes declines". It is with great passion and desire that the poet says--"But thy eternal summer shall never fade". Poetry becomes, to him, a way of accessing this immortality.

In the last two lines, Shakespeare does talk about the eternizing power of poetry; its ability to outlive time and succeed in its acid test but as far as humanity is concerned, I do not think he ensures any note of immortality. What he does is to mark the limits of eternity with the limit of human survival on this earth. As long as there are human beings upon this earth and as long as they are receptive or sensitive enough, the poetical lines would be remembered and in it would survive the beauty of the fair friend. That is his argument.