“The other issue — we don’t really have a good enough model for a single biological neuron, so it can only ever be an approximation given the neuron model we think is correct.”

“Fine.”

“A colleague of mine, not a close one, a fellow named Henry Markram — he got a ton of EU funding to basically emulate the processing ability of cortical columns.” You stopped and held up a card of a naked cowboy on a horse. “How’s this one?”

“Mom.”

“What? It’s funny.”

“That card’s for gay men and middle-aged women.”

“Well, I don’t know, they have nothing here.”

“The processing ability of cortical columns —”

“And um —” You flipped the card over a few more times as if not quite ready to put it back. “Markram theorizes the human brain has a processing rate of an exaflop, which is basically —”

“A what?”

“— about one million teraflops.”

I shrugged and shook my head.

“A teraflop is a trillion operations per second. So it’s a million times a trillion operations a second. What about this one?” You held up a card with a penguin wearing a birthday hat that said, Well, waddle you know, it’s your birthday.

“How old is Luke?”

“Twenty-one.”

“That card’s for a five-year-old.”

“I thought the penguin looked kind of sardonic.” Which was true, it kind of did. I suggested a card with a Facebook thumbs-up that read, Happy 24 Hours of Constant Notifications Day.

“Really? A Facebook joke?”

We eventually settled on one that read, If you were Jesus, today would be Christmas. An appropriately wry millennial sentiment. The baby card was much easier to find. “They’re all the same,” you said, meaning either baby cards or babies.

As we waited in the checkout line I mentioned that I’d been reading about “the singularity” — the moment artificial intelligence would reach a critical mass of self-augmentation and exponentially supersede human intelligence. You nodded, nonplussed.

“This is, of course, presuming that computers will ever think as humans do,” you said. “That computers can be people.”

“And will they?”

You scrunched up your mouth and shook your head. “Not as long as we’re using digital computers.”

I watched as you began fishing for your wallet in your purse. How did I still know so little about your work? Your life? Our conversation felt tinted with the colour of loss, the loss of detail and intimacy that comes with a couple of Skype calls a month. After years of trying to push myself away, I suddenly felt the distance I’d managed to put between us. The Atlantic ocean. Six time zones.