Optimum: Most favourable or desirable, best.

Dear Chuck,

Our son was born today, and Serena thinks he looks like me. She's walking him up and down the hall and you're asleep on the fold-up next to me – you were up all night, don't worry about being tired. I saw how afraid you were at the hardest parts, but you didn't let go of my hand. That's how I knew you'd become a man, and that's a fact I've known for a long time now. There have been times when I wasn't sure if you would live this long, but if I'm sure of anything about you, it's that you're vital: you've tried so many of the things that life has to offer that there's a current running through you, a kind of heat and energy that means when I touch you it's like wrapping my hand around a live cable.

I almost want him to cry, so Serena will bring him back to me, so he can settle in the front of my gown where he belongs – but I have to get this down before you wake up. Henry, this is your mother. Behave yourself for five minutes while I work out what I want to say to your father.

As vital as you are, I've almost lost you at least twice. It was too late the first time, and I knew you were alive, and I tried not to care. The second time, my heart stopped. There's a line in a poem about all the clocks stopping together, and losing you was like that. The world began to freeze and nothing it ever did would interest me again. It terrifies me to think of all the times I could've lost you before you were really you, before there was an us: an accidental overdose, a game gone wrong, a collision in a far off country. I would've cried at your funeral because you were my friend, but despite the empty seat I wouldn't have known that everything had gone wrong.

I wouldn't have known that you were supposed to be my heart, my other half, my drive. It isn't so much a place in my heart as my whole heart, since it stops when you stop and only beats when we're linked somehow, even by the most tenuous of threads.

Our newest thread isn't crying yet. I see he takes direction well.

The obstacles we've had to overcome are innumerable, and you and I are the least of them. Now, I feel the fact that we would love each other was a certainty, although it didn't feel that way at the time. That's why everything was so dramatic, why we made so many grand speeches even though we were so young. I wouldn't change it, though. I've realised that now. Simple love is not for us because we're not simple people. Simple people grow up, graduate, get married and have babies at set times. They get bored and have affairs, or they don't. They change jobs and relocate, they buy fast cars when their hair starts to thin and their clothes get too tight. We're not simple people.

Our son was born today, and he is not a simple thing. He was born out of pain, and struggling, and casting about for the right way to do something everyone says is natural. We were born the same way, the concept of us, and in the end I find us – and him – the most natural thing in the world.

Don't you dare read this while I'm in the room. I don't want to cry or lactate, which will happen if I cry, because then Henry will cry, and then I'll assume he's hungry and all hell will break loose. Read this while you work on the Empire: it's the second most beautiful thing you've ever made.

The first most beautiful thing has been fussing, apparently, so I've tucked him under my arm as I write. Serena is getting coffee, and I imagine you'll wake up when you smell it, determine it's not French roast and say something disparaging. Thank you for not drinking it in front of me these last few months. I know you've been having secret espresso dates with Nate. I don't mind.

I love you. Congratulations on becoming a father, and thank you for being the very best husband, the very best friend, the very best ally I could've chosen. You will only continue to excel at being a part of our family.

Yours, as always,

(Special thanks to Henry Bass for being good while his mother wrote this, and to Auntie Serena for the tissues).