So the potatoes stacked high in his kitchen window don't bother me any, as I wander around the back of his house, hunting for some sign of his whereabouts. We're supposed to be having a meal together, but he is never easy to find in the sprawling old Bearsden bungalow he has occupied for years.
A woman once shared the house with him, but she is long gone. She was a police officer, not a profession noted for original thinking, and the disparity grew too great in the end. So they drifted amicably apart. Al does everything amicably. Yet I've seen him drive people insane.
Once he has figured something out he will not budge. He has anticipated your arguments and refuted them in his head. "Doing the sums," he calls it. But not being combative or fast on his feet, this can come across as stubborn and pig-headed.
At his work, as a marine engineer, his creative thinking saved lives and millions of dollars. Now in his retirement it makes him grow big fat vegetables, worry about climate change and exercise his sideways sense of humour on unsuspecting strangers.
"I hardly ever drive these days," he tells me, when I find him behind the broccoli in his garden, and ask why the car in his drive is laced with spider-webs. "I've done the sums. I don't need to. Five minutes walk away is an Asda's, two Indian carry-outs, a fish and chip shop and a brand new Alzheimer centre."
"That's handy," I say, taking the huge heads of broccoli he hands me and accompanying him to his kitchen.
"Very," he says. "First time I saw it, on the way to the supermarket, I went in and had a look at their leaflets. Got talking to them. I asked what they did and how long they'd been there. Said I hadn't noticed them before.
"On the way back from Asda's I called in again, had a look at their leaflets and got talking to them. I asked what they did and how long they'd been there. Said I hadn't noticed them before."
"I had an aunt who got Alzheimer's," I say, stepping over buckets and basins bulging with big red potatoes, and finding a small space to stand. "I phoned the Society and they tried to be helpful. But there's not much anyone can do. You think you'll catch it?"
"Probably," he says. "You know why I was a good engineer?"
"You'd a brain bigger than ten of these potatoes," I say.
"Certainly," he says, taking the heads of broccoli from me and piling them in the sink. "But also I'm aware of something other people keep forgetting."
"Which is?" I say.
"The world is not organised for our benefit. Things go wrong. It's the first principle of engineering. Shit happens."
"Doesn't make you a cheery companion though," I say, trying to catch sight of him beyond the stacked pyramids of potatoes.
"But it means I prepare for every possibility," he says. "I do the sums. When I retired I put my money into National Savings. Everyone said I was stupid and should invest in shares. What happened?"
"They lost their money in the crash and you've still got yours," I say.
"Correct," he says.
"But what's the use of sackfuls of cash you don't spend and have no one to leave to?" I say.
He goes silent. "I have someone to leave it to," he says, not meeting my eye.
"The policeperson?" I say. "Have you two got back together?"
"Don't be daft," he says. "Getting back with your ex is like breaking into Alcatraz."
Again the silence, stretching. "I'll tell you sometime," he says. "Maybe."
"Fair enough," I say. "Now about these root vegetables. What's the plan? You already have enough to feed a medium-sized medieval village."
"Well, I never turn the heating on in here," he says. "So these potatoes will feed me for five years. I've done the sums. Whatever happens - power failure, premature senility, the end of civilisation - I will survive."
"In here with your potato mountain?"
"In here with my potato mountain."
"All you need is Felicity Kendall," I say.
"Only if she doesn't eat," he says. "I didn't include her in my sums."