Schoolteaching is a profession, but it's not like becoming a doctor or a vet. No one would want to be operated on by an amateur who hadn't had years of experience. The prospect of going to the dentist and being confronted by somebody with a lifelong passion for teeth but no university background or training would alarm all but the most steely. For that reason, there is no Teeth First, though we do have Teach First, albeit with intensive training.

Nick Clegg and others who argue that teachers must first be qualified are fundamentally misunderstanding the nature of the profession. The teacher's role is much more akin to that of a parent. It is a great loss that governments worldwide have made teaching much less like being a parent than an impersonal civil servant. No job is more important than parenting, yet no one is suggesting parents go off for a university course to qualify as a parent. Parents pick it up as they go along, and that's exactly the way great teachers are forged.

There is one fundamental difference between parenting and teaching. The former are self-selecting, whereas the latter have to be appointed by those with knowledge and experience. I write this from a conference on Education for Tomorrow run by Singapore Management University. The principals attending are from schools around the world, state and private, and almost all agree that great teaching is a gift that some have, and others will never acquire even if they spend 10 years locked away in a university. Most of us can tell within minutes whether someone has "got it" or not.

What is the teacher X factor? It is simple: passion and intellect, heart and head, love of subject and love of children. Selfless compassion and tireless energy are not enough, however. Successful applicants need deep subject knowledge and the ability to communicate in ways that will inspire the young to want to learn.

Those who care more about themselves, are time-watchers, and place pay and conditions above caring for the young will never make it. Teaching is a vocation as well as a profession.

Rookie teachers have much to learn. But they will learn much better on the job than at a university. They learn by reflecting on their own experience of the teachers who inspired them, and by watching inspiring teachers in the schools to which they are attached. The best teachers I have known are constantly learning. Not to want to learn is both a shame and shameful, because no profession on earth has as much power to change large numbers of lives for the better. The very best teachers I have ever known are as eager to learn on the day they retire as on their first day in the classroom.

I must declare an interest. Wellington College is a teaching school, where we work with a dozen state schools to train new entrants into the profession, and help them develop professionally. I wouldn't want to see university training disappear altogether, but I'm glad that the bulk of training is now being done on the job. Showing trainee teachers the ropes is equally invigorating for experienced teachers. It helps them to keep their edge, encouraging them to reflect on their own practice, an essential prerequisite for all great teachers.

So Clegg is plain wrong. But if he loses his job at the general election, I would certainly offer him a post, and not insist that he lose a year becoming qualified.