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Howard Jacobson: When ordering a salt beef sandwich, beware the moral minefield that awaits

It's a contradiction of Jewish law: the more you eat kosher, the more of a pig you make yourself look

Every man is tested in his own arena of resolve – the marriage bed, the debating chamber, the athletics track. In Bernard Malamud's novel The Natural, the baseball field becomes an allegory for life itself, where good and evil contend for the player's soul. Martin Scorsese gave similar weight to the boxing ring in Raging Bull. No matter how material and physical the contest, what is actually under scrutiny is the spirit. Too big a schmaltz-fest for my taste, all that weeping and wilting when Chelsea lost the lot recently. But matters of the heart – love, bereavement, pride – were undoubtedly in play, far more than a footballer's ability to stay on his feet in front of goal.

Thus the great tournaments of art and literature, Tolstoy's Borodino, Shakespeare's Agincourt, and, for many, Blair's Iraq – in these, the spiritual worth of men is weighed in the balance and it is as moral heroes or moral cowards that they emerge. My arena of resolve would appear to be the salt beef bar.

There is a specific salt beef bar I have in mind – the Brass Rail at Selfridges – but since it's as a salt beef bar of the soul that it tests me I accept it could be anywhere. Not quite true, allowing that the Brass Rail's salt beef is the best in London, and the quality of the beef itself is not incidental to this parable. But I don't want readers to become fixated on place. My subject is the animating principle of man, our vital essence – what Hindus call jiva and Jews neshomeh.

I go to the Brass Rail every couple of months with a new friend, a once-famous show-business journalist whose columns I read avidly in the Daily Mirror when I was growing up, a man who routinely danced cheek to cheek – that, anyway, was how I imagined it – with Marilyn Monroe and Sophia Loren. Would such opportunities, I used to wonder, come my way when I became a writer. Now I am a writer I have a retrospective answer for myself: no, they wouldn't. But at least, as we sit talking and remembering over salt beef sandwiches in Selfridges, Sophia's perfumes waft my way.

He is a couple of decades older than me, my new friend, age being relevant only in so far as it explains why I do the queuing for the sandwiches, his always without mustard but don't skimp on the pickle, mine don't skimp on anything. Although these are not sandwiches on the New York scale – that's to say they are constructed to be held in the human hand – a certain amount of spillage remains de rigueur. A salt beef sandwich is not a salt beef sandwich if you can bear to look at another person's face while he's eating it. In fact this doesn't apply to my friend who, as he says himself, is past the gluttonous age, but it does apply to me. Think of it as one of the contradictions of eating according to Jewish law that the more you eat kosher – all right, kosherish – the more of a pig you make yourself look.

For the very reason that a salt beef sandwich must be plentiful and, of course, moist, I become agitated when I queue for ours. Depending on the time of day, Selfridges has two or three cutters on duty. So that's two or three sides of beef you are eyeing while you wait – eyeing for colour and consistency, for you no more want it crumbly than you want it dry – and two or three cutters – eyeing them to decide who is the more expansive on the day, the more likely, for whatever reason, to pile your plate to overflowing. Should it appear that one of them is loading sandwiches less generously than the others, it becomes a matter of extreme urgency that you don't end up being served by him.

To be certain you get the most munificent cutter, and that his beef is the most moist – which is by no means a sure concatenation – you must resort to underhand methods. You gallantly let the woman behind you in the queue go first, you pretend your mobile phone has just gone off and step aside, you lose your wallet, you feign a heart attack. Several times I have quit the queue altogether and returned empty-handed to my friend in the spirit of a failed balloonist, dispirited but promising to make a second pass at it when conditions are more auspicious.

Gender enters into this as well. When it comes to serving you salt beef, it stands to reason that a man will be more gargantuan than a woman because only a man knows how much of it another man can consume. Last week I made a couple of spectacularly ingenious manoeuvres to evade the female cutter, only to see the male cutter I'd secured remove his surgical gloves, nod to me in apology, and cede his position to a second woman. Too late now to be seized with a palsy, I muttered my order in a spirit akin to despair.

But this was just the beginning of my trial. The replacement cutter was not, as it turned out, in the slightest bit niggardly. She was, however – at least to my eagle observation – inconsistent. On one plate a salt beef sandwich fit for a king, on the other for an emperor. A moral struggle distorts time. For what must have been no more than a fraction of a second measured temporally, though it felt like an eternity of thought, I hovered between greed and generosity. Would I keep the emperor-sized sandwich for myself, or present it to my friend? My need was indubitably the greater. I was bulkier, hungrier, more desperate. He always leaves some salt beef on his plate. I polish off every fibre. To give him the bigger plate would have been a waste; no, given the millions of starving people on the planet, more than a waste, a crime! But I was the giver on this occasion, he the receiver, and by every law of hospitality a giver must never favour himself.

So how did I do? Did I emerge from this ethical tussle with any credit? Reader, I will never know. Mustard not morality decided it. Yes for me, no for him, and once she'd squeezed it on one sandwich and not the other the matter was settled. It fell out, I'm pleased to say, as it should have. My mustarded sandwich was the one for the king, my friend's for the emperor. I'm still thinking about how much of it he left, but without bitterness. That, at least, I call spiritual success.