"Can I come?" he adds.
"You wouldn't want to," I say. "It's a Steve Gadd and James Taylor gig and you don't like either, far as I know."
"Who's Steve Gadd?" he says, which doesn't surprise me. Al has strong but not varied musical interests. He's into hard rock and gutsy guitars. He's a Hendrix fan.
"Probably the best drummer in the world," I say.
"I'd go as far as Amsterdam to avoid listening to a drummer," he says.
"But you wouldn't fly there, would you?" I say. "You're scared of flying."
"You know that's not true," he says, seeming peeved. "I'm scared of climate change. I don't believe anyone should fly anywhere."
"You think burning your passport will save the planet?" I say.
"You think not eating meat will save the animals?" he says, and it's a fair point. But I have an answer.
"You're right. We both are. It's the same principle. You decide between right and wrong by imagining how things would turn out if everyone did what you're thinking of doing - or not doing. If the world would be a better place, it's the right thing to do."
"Or not do," Al says.
"Or not do."
"Listen," he says, grabbing my arm, as we turn into his drive. "Could you shut up a minute?"
But I'm already in full pontificate mode, so he might as well wave a white hanky at a charging bull. "There's an alternative principle," I tell him. "The greatest good of the greatest number. Politicians use it to justify everything from benefit cuts to world wars. It's the source of all evil, because even the simplest dynamic system - as you and I know - can produce totally unexpected behaviour.
"A world full of people is anything but simple. So believe your model, focus on ends and you can justify any means you like. That's not ethics. To be ethical you have to care about what happens to Mrs McGinty, three doors down, not to the aggregate of 60 million faceless units in a mathematical model that deludes you into imagining you can tell the future, when you're actually talking spurious, self-satisfied, evidence-free, unmitigated shite."
"Has the wee spring wound down yet?" Al says, when I stop to draw a big breath.
"Just about," I say, as we take a seat in his back garden and contemplate a vegetable patch the size of Hampden Park. Years of Al's tender care and scientific nutrition have given it sinister strength and an air of brooding menace.
His broccolis are the size of village bus-shelters. The regimented rows of tall leeks look set to invade Czechoslovakia.
"Just about," I tell him. "Has anyone wandered into your vegetable patch and never come out again?"
"Not yet," he says. "I think it might have eaten next-door's dog though."
"Any chance of a coffee before I get back to work?"
"Listen, what I was trying to say when you were off on one was this," Al says, five minutes later, placing two full mugs carefully between us on the wooden bench, dotted delightfully with bird-shit. "Isn't this blog supposed to be mildly humorous?"
"That's the idea," I say, sipping his black, acrid coffee and trying to tell my face it's nice.
"Well I've gone over this conversation in my head and it's mainly you on a moral philosophy rant. That's too serious for your readers, isn't it?"
"Most of them," I say. "It wasn't the plan. I intended to talk about orgasms."
"Ancient history then," he says. "But why? Is there some topical science angle?"
"There sure is," I say. "Couple of Australian scientists have launched a study to find out why people fake their orgasms."
He shakes his head. "Pretty obvious, I'd say, even for Australians. Daft bloody scientists."
We sit in companionable silence for a while.
"You ever fake an orgasm?" Al says, without looking at me.
"Nah," I say, staring straight ahead. "You?"
"I can't remember," he says and we sip our coffees, as the vegetable patch rustles ominously and a plane passes overhead, bound for who knows where.