Shakespeare has truly given immortality to the subject of his? ardour in the poem Sonnet 18. In a similar way in which the artist potter gave 'immortality' to the two young loves in 'Ode on a Grecian Urn' by John Keats, Shakespeare has enshrined the presence of the subject in words as a work of art - a beautiful work of art- true to his comment...
'so long as men can breathe and eyes can see
so long lives this,and this gives life to thee'
... we really do remake,if not bring back to life, the beauty of the person he describes. He is saying that love transcends and outlasts death - the fact that he loved the person can never be changed, even by death bringing to an end both of their lives - the love they had still remains in Time.
I think that the last two lines are saying that this particular poem (and great art in general) will live on and that she who is in the poem will therefore live on as well. I don't think he is really saying mankind is immortal -- just those who are enshrined in great art.
But I think that there is another point, which is that the things that make her beautiful to him are eternal. Obviously beauty is not. But I think he is saying that love and internal beauty are eternal.
So, I think what is eternal, according to this poem, is A) art and B) love and being loved.
The theme of Shakespeare's "Sonnet 18" is that his lover is more beautiful and desirable than "a summer's day" because even such a wonderful season like summer has its flip side-it's too short and sometimes too hot. He concludes by saying that he wishes to immortalize forever the beauty of his lover in his poetry.
The theme of Shakespeare's sonnet is that mankind is indeed mortal - all human beings will die. The only way to immortalize a human being is by praising him in excellent verse which the future generations will always read. Shakespeare compares his lover to "eternal summer" and he has immortalized his lover in his sonnet 18:
But thy eternal summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou growest:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
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.