We highest primates are past-masters at inventing words that let us off the hook. For example: most of us spend many of our days, and the odd sleepless Sunday night, in a mixture of vague dissatisfaction with our work and pay, niggling worries about our relationships and health, and low-level fear about our jobs and mortgages. Trouble is, it is very hard to admit this while keeping up that self-regard which is, it seems, so vital to human life. So, instead, we call it stress.

And when we ask why we put up with this stress, we have another cute word ready and waiting: after all, it is our career we are talking about here. Everybody knows that you have to make tactical compromises when it's a question of your career. And because everyone else accepts this argument, you can, too.

But this is the whole cunning of The Career, which is actually the Third Horseman of middle age. Unlike The Baldy and The Gut, its power comes not from brute physicality, but from its being the master of disguise: "career" is one of those slippery little words that have changed their meaning, like smart viruses, to entrap us. Words like "middle-class" and "executive".

When Marx was around, "middle class" meant people like Engels, who owned factories and multiple houses: rich, but not nobs. Even in Ted Heath's dog-days, "executive" still meant someone who moved and shook. Nowadays, any C1 is sure they are "middle class", "executive" means a four-bed Wimpey or a tarted-up Mondeo - and everyone who wears a suit to work (because they are told to) has a career.

Until the 19th century, career meant something to do with galloping or zooming about, as in horse or comet or Porsche Carrera. It only started applying to people's CVs around Napoleon's time: the first career in European fiction is Julien Sorel's in Scarlet and Black (Ewan MacGregor's first big TV part: my wife put him down for stardom right back then, at the mere glimpse of those then-plump wee buns). The Revolutions (French and Industrial) had blown the old order away, and one of the images people hit on to describe the new world was that of someone careering to quick, new wealth and social status.

A career was nothing to do with paid work - in fact, it was the opposite: the man who "made a career" implicitly jumped his way up by using shady dealing, ruthlessness and arse-licking. Indeed, the phrase still suggests this in French and German. Ah, so maybe that's why they call them Carreras.

No, let us be honest: a hundred years ago, no one would have called our kinds of jobs careers at all. They would have called them situations. As in static. Or comedy.

Our mortgage-servicing day-jobs (as we ought to call them: the Germans call them bread-jobs) are rarely anything to do with dynamic movement. Not one in a hundred of us will career up to the escape-velocity we dream of and burst through into the mythical freedom which is constantly held out as the goal of our strivings. Our work can only deliver more of itself: the harder you graft at teaching, lawyering, doctoring, accounting, managing or whatever, the more you will get of it to do; end of story.

So here is the hideous question that the Third Horseman whispers in your ear as you stare at your Baldy and your Gut in the all-too-clear mirror: "Observe, O ye of three decades and then some, the pitiless dates already inscribed in next year's filofax refill; tell yourself that what you are doing workwise now is more or less what you will still be doing when your mind starts to turn (or wander?) towards the retirement bungalow. So how does it feel?"

I have two friends, one Welsh archaeologist (a lovely man) and one Geordie personnel hatchet-man (an evil bastard) who are truly doing what they like, and could thus answer: Great! But as for the rest of us, an honest reaction would be: So how the hell did it come to this?

The answer is simple but nasty, like jumping off a bridge is simple but nasty: somewhere between 27 and now, you got lazy. You got used to the monthly hit of cash and the paid-for hols and the pre-sorted pension-scheme and the car and all. You never really thought twice about it because, after all, it was a career, wasn't it?

So now you find yourself not only bald and fat, but careering up the wrong motorway with no exit ahead until that last Great Interchange where all roads meet and all career paths get overgrown.

And then the voice of the Geordie hatchet-man comes to you, saying: Don't forget, we're all only life-support systems for our DNA.

Of course! we cry, and overnight the waiting-rooms of clinics are crammed with ex-lads and their partners anxiously consulting harried docs about sperm-counts and tubes, suddenly desperate in case we have missed out on vicarious immortality by putting off breeding for another year in favour of the alleged career.

Which next week brings us, kicking, screaming and filling nappies, to the fourth and final Horseman of Middle Age