From the moment I decided I wanted to be a teacher, I have been fascinated and tantalised by which people make outstanding and successful teachers. It was obvious it had a lot to do with their knowledge and expertise in enabling the pupil to access something of significance.
That there was something else - something indefinable and elusive about the personal qualities, habits and dispositions - was also clear. I guessed, from my own experience of secondary schooling as a pupil, that there was also a "school effect" on the likelihood of teachers being successful.
It was in Wales that I first encountered an outstanding teacher. Margaret Lewis taught in a primary school in Croesyceiliog, Cwmbran.
As her headteacher said: "All the children in Mrs Lewis's class learn to read. I don't know what it is about her but she gets the best out of the children - all of them."
She was warm, cheerful, optimistic and energetic. She had learnt the skill of doing things simultaneously during a lesson. Her classroom was smoothly organised and had the sort of display that celebrates each child's progress, asks questions, reinforces languages and maths, and is beautiful.
But then all successful teachers do that.
Ms Lewis also effortlessly practised what on the other side of Offa's Dyke the government calls "personalisation".
It was easier for Mrs Lewis than her secondary colleagues. Often they teach 200 to 300 difficult youngsters each week. They spend a lot of preparation time in the autumn putting names to faces so that they can use the names in greetings in the corridors at lunchtimes, as well as in lessons.
They deploy marking practice that makes students feel special by providing extensive private written feedback to each student at least a twice a year, and they are masters of what we now call "assessment for learning".
Mrs Lewis and her successful secondary counterparts also shared the unpredictability of teaching. Something worked nine times out of 10 and then it didn't. And that is where the first definable feature of her specialness surfaced. She never despaired. She saw the child's failure to learn not as their inability to learn but as her failure to teach - to find a way to overcome the pupil's learning blockage.
She had other beliefs too. She saw education as the bedrock of social justice, political freedom, and the individual's freedom as an adult to argue a case which is just. She believed that pupils showing great effort was not an indicator of their limited ability to learn but of their character, which would enable them ultimately to overcome obstacles to their learning.
She talked about "her" children, not "these" children. It never entered her head to put a limit on what children might achieve, however dire and challenging their home circumstances.
"What more can you expect from children from backgrounds like this?" never crossed her mind.
In short, she believed in the transformability rather then ability of children. And she believed, in a way that communicated itself to all her children, that she could effect that transformation.
Wouldn't it be wonderful if there were suddenly a multiplication of Mrs Lewises? There are many in-school and whole-school factors that make that more likely.
How heads take staff meetings, parents evenings, award ceremonies and assemblies clearly affects disproportionately the climate of the school and the way their teachers work.
I have worked in Birmingham and London, where those schools which succeed against sometimes formidable odds are led by people who, like Mrs Lewis, believe in the unlimited educability of all their pupils.
They have a clear value system which rings true in their actions. They are on the gate, in the street, and ferry to school for revision those who don't get up in the morning. They inspire by example. They, too, make the whole educational process personal.
Second, in the matter of professional development, teachers have moved on from talking to each other about teaching to involving all the staff and pupils. So staff talk about learning and teaching; observe each other's practice; plan, organise and evaluate their work together in teams; and learn from, and teach, each other.
And third, in outstanding schools, the bottom-up data about children, their background and their stage of development, help their teachers' hopes for modest and ambitious progress that translates into targets which are the stock-in-trade of teachers who embrace formative assessment.
These are complemented by the wider knowledge of other schools and other teachers' achievements, either with the same or similar youngsters.
So those are the circumstances in which a Mrs Lewis prospers, where her example leads to other teachers having the courage to dare to evaluate their practice and, in doing so, change society. It is these teachers that I salute and celebrate. They are the architects of tomorrow.
Professor Tim Brighouse is London commissioner for schools. This is an edited version of the General Teaching Council for Wales lecture that he delivered earlier this month