What do you do or say when you don't want to agree with someone trying to convince you of something? Look downwards for longer than is usual? Grunt, stare at the curtain rail, clear the gunk from out of your thumbnail and recite inwardly and silently: "Tomorrow and tomorrow and tomorrow creeps in this petty pace from day to day…"? I think I have something that can better all these. A failsafe backhander for every kind of situation from the personal to the political and back again.

   Take the face-to-face barney. Thanks to the work of the world's psychologists, psychotherapists, psycho-analysts and counsellors there is now a huge literature on the whys and wherefores of human behaviour. Thanks mostly, but not entirely, to the wisdom of Americans, this literature has been boiled down into useful catchphrases: 'in denial', 'neurotic loop', 'dealing with stuff', 'feeling grounded', 'why are you resisting this?', 'your shadow', 'it's the father speaking', 'poor object relations', 'dealing with your unconscious' and so on. Now it's one thing for psycho-practitioners to use these terms as reference points in their work, but it's quite another when they are taken up as weapons in the daily battle of human relationships.

I'm only guessing but I would think that many of you reading this will either have been on the receiving or the giving end of some of this stuff. Or both. Part of the game in swapping these psycho words is who's got the quickest trigger finger:

"I didn't say that."

"Neither did I."

"You're in denial."

"So are you."

"I said it first."

Another part of the game is to come up with a level of psycho knowhow that you hope will be of a higher level than the person you're arguing with:

"You're not looking at your own shit."

"You're stuck in the 'Mirror Phase'."

In a different kind of discourse, you may well find yourself in an argument about Iraq, the railways, teenagers, God or the best ever bad movie. As we drift easily between attitudes, opinions, facts, disputed facts and assertions, quite often a moment will crop us where we feel that no matter how vehemently or certainly a particular point has been made, there really is no hard evidence that it's true:

As I write this, I'm being told over and over again that Saddam Hussein is a danger to my security. Similarly, I've been told that the railways are getting better, teenagers talk in monosyllables, God exists and 'The Blob' is the best ever bad movie. Maybe all these statements are true, but if you don't think so you should consider your options.

Lumping together the psycho talk with the politico and the arty talk, what we're dealing with here is the person leaning into you making assertions. Of course, you may feel like standing your ground and boxing. You may feel like running away. You may feel like practising dumb insolence, and, as Adrian Mitchell put it in his poem on the subject, "thinking of sick". But there is another possibility. It is using, with discretion — (remember its infrequent use will make it all the more deadly) — one simple two-word phrase: "Or not."

Here are some instructions on how to use this dangerous weapon. Don't shout it or say it in anger. Try to leave a two-second pause before saying it. Use it with a very slight upward inflection, as if it were almost but not actually a question. Follow it with an interested look straight into your co-conversationist's face. Try to keep very still throughout. If you need help with this, think carefully where your hands are and leave them there.

As I suggested, don't overuse "or not". It will quickly lose its deadly powers. It should be saved for a crescendo moment, a moment that your co-discussant thinks is some kind of coup de grace. As in eg:

"And I'll tell you something else: your problem is not just that you haven't come to terms with your inner child. It's the little girl inside who talks for you."

(Two second pause, raise eyes, look.)

"Or not."

(Whole body still.)

"Well, if what you say is right, then we've got nothing to fear from Saddam and we can all go home and sleep in peace. But I'm telling you that Saddam can't be trusted and no matter what the inspectors do or don't find, he's a direct threat to this country."

"Or not."

"God exists."

"Or not."

I'm telling you, this little phrase is a failsafe, perfect rejoinder.

Or not.