A J Ayer was once rude to my mother. I'll break off there for a moment, because that affords an opportunity for one of those rare moments when this column offers moral instruction.

I suspect that most people have an unarticulated moral code according to which you can be as nasty as you like to someone you're meeting for the first time and will never meet again. People with nice respectable faces used to shout the most extraordinary things at me in the years when I was cycling around London (and this can't be blamed entirely on my provocative behaviour).

But I feel this must be wrong. There is almost a spiritual sense in which it is bad that a person's idea of you, based on the one encounter with you they'll ever have, should be of someone grumpy and rude - even if they don't know your name and forget about you five minutes later. If you are famous, there are more practical reasons for behaving decently. Being rude to a stranger is like creating a little wind-up doll who will wander the world and whenever your name is mentioned will pipe up with: "Well, I met him once, and he was a real bastard." If you are really famous and if what you do is interestingly nasty, then if the victim tells ten of his friends and they each tell ten of their friends and so on - well, do the arithmetic yourself. Pretty soon the entire world will hate you.

I've got nothing particularly against William Golding, for example, but on the only occasion I ever encountered him he yelled at me down the phone, and I was still young and easily upset. The one time I met Harold Pinter he just turned away without a word, though admittedly on the scale of Pinter's reported social behaviour that counts as cheerful friendliness.

Ayer wasn't rude to my mother for any interesting reason. On the evidence of Ben Rogers' new biography of him (Books, page 48), my mother's failure to be titled or a TV celebrity was probably sufficient provocation. He was not a man whom people told nice stories about. In a TV review, Clive James described an appearance on a programme during which he "indulged his bad habit of saying 'Mm, mm' impatiently while other people spoke, as if their points were too obvious to require putting. I found this wonderfully unendearing."

Maybe this man who made his philosophical reputation at the age of 25 just thought he was cleverer than most other people he met and didn't trouble to hide it. I once saw him on TV getting very huffy when a panel of reviewers that had just praised The Two Ronnies Joke Book dismissed his memoirs as dull. On the other hand, Martin Amis wrote amusingly about giving him a lift in his car. Ayer was trying to light his pipe and when he had finished he placed the first match not in the ashtray but on the knob of the gearstick. With the second match, he looked around and then balanced it precisely on the edge of the key in the ignition, where it remained for a full second or two before falling off.

Ayer despised most other philosophers as well. In the introduction to his famous first book, Language, Truth and Logic, he claimed to be "providing a definitive solution of the problems which have been the chief sources of controversy between philosophers in the past". But there was a problem and, to do him credit, when asked 40 years later what were the main defects of "logical positivism", he replied, "I suppose the most important of the defects was that nearly all of it was false." And indeed Anthony Quinton, one of the senior figures in British philosophy, is quoted in Rogers' biography as saying that he made no positive original contribution to philosophy at all.

This seems startling but it could be said of the vast majority of philosophers. Bernard Williams made an amusing defence against the attack on contemporary moral philosophy of the "linguistic" style for being peculiarly empty and boring: "In one way, as a particular charge, this is unfair: most moral philosophy at most times has been empty and boring."

In fact this column can now offer a second piece of advice. Careers advice, this time, for any young people who may be reading. When considering a career, imagine whether it's still worth doing if, like most of us, you're not all that good at it. A second-rate doctor, ticket collector or dustman remains useful. A philosopher who turns out to have been wrong is just a waste of space. And being rude while you're at it just makes things worse.