When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty’s field,
Thy youth’s proud livery so gazed on now,
Will be a tattered weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
To say within thine own deep sunken eye,
Were an all-eating shame, and thriftless praise,
How much more praise deserved thy beauty’s use
If thou couldst answer “This fair child of mine
Shall sum my count, and make my old excuse”
Proving his beauty by succession thine.
This were to be new made when though art old,
And see thy blood warm when thou fall’st cold.
Here the motivation for children is laid out plain, and the next midnight feeding, teen-aged eye roll, or the piercing public voice saying the most embarrassing things will help you remember. Children are for your old age. Your children will reflect your beauty, remind you of your lusty days. When your brow is wrinkled, their youth will make you feel young. They’ll warm your blood when you’re old and cold.
The first four lines set the scene. Using farming as a metaphor Shakespeare defines the brow as a field. Winter (old age) is digging deep trenches in the field. Youth’s livery will be a tattered weed. Farming, like the seasons in the first sonnet, is a good metaphor for the life cycle. The seed can be the unborn infant; the seedling the youth; the mature plant the young man; the dying plant the old man. Of course even the urbanites in Shakespeare’s time were closer to the farming life cycle than we are today. This metaphor would have been obvious to anyone reading in Shakespeare’s day. When you’re old people will ask you: what were you like when you were young? I don’t think Shakespeare means any hypothetical loving grandchildren here. I think he means the judgmental societal matrons who will weigh your worth by who you married and what he did and how many children. These are people asking the ancient question: “Why should we haul your decrepit self around, care for you when you can’t? Of what worth are you to us? Why shouldn’t we leave you to the wolves?”
Shakespeare concludes by saying if you had a child, this concern would be moot. Any child of your would be as pretty as you were and obviously your child. He shall stand for you, “sum your count”, and warm your blood. Because your child is of you, he shall be you.