Sometimes the simplest questions in life are the hardest to answer.
For all of the millions of pounds invested in researching school effectiveness, and the thousands of hours spent by policy-makers reforming education systems, do we yet have a unanimous answer to this most important of questions: "what makes a good teacher?"
The short answer is "no".
But this week saw a significant move towards an evidence-based view that might yet influence the politicians.
At the invitation of the Cambridge Assessment agency, a group of experts gathered at Westminster to pool their research knowledge and grapple towards a definition of a "good teacher".
The timing was excellent since the House of Commons Schools and Families select committee is about to start an inquiry into teacher training.
And it was encouraging that its chairman, Barry Sheerman, who chaired this seminar, said his committee preferred to be informed by evidence based on thorough research rather than on opinion.
The timing was good in another way too.
Ofsted has just issued a report praising the innovative teacher-training programme, Teach First.
This scheme places high-quality graduates straight into challenging secondary schools for two years.
In this way it offers practical and hands-on training much earlier than in a traditional teacher training course.
According to Ofsted, the Teach First scheme is both producing a very high proportion of "outstanding" teachers and is also helping to transform the inner-city schools where they are being trained.
It also attracted graduates who might not otherwise have considered teaching.
So this is a good moment to reassess what it is that produces good teachers.
This question also relates to some of the reaction to last week's column, when I wrote about research that found independent schools were recruiting a disproportionate number of the "best" teachers, as defined by those with higher degrees.
A number of respondents took issue with this definition of what makes the "best" teachers.
I should say here, in defence of the researchers, that they used this measure because it was the only one that they could quantify for statistical analysis.
'Soft and fluffy'
They would agree, as would I, that teachers with the highest qualifications are not automatically the "best" teachers in the classroom.
Having got that off our chests, let's turn to what the experts were saying.
Professor Patricia Broadfoot, a former Professor of Education and now vice-chancellor of the University of Gloucestershire, argued persuasively that the evidence from international studies showed that "the highest quality teaching and learning comes when we have the greatest autonomy for the teacher and the learner".
The good teacher, she went on, was someone who was "left to get on with what they think their students need".
This certainly sounded like a rejection of the prescriptive approach of the national curriculum and the numeracy and literacy strategies. Professor Broadfoot went on to propose a much more child-centred approach.
While insisting she was not advocating a "soft and fluffy" style of teaching, she argued that research showed that a good teacher had to engage with "the powerfully charged emotional relationship between teacher and pupil".
So, for Professor Broadfoot, the key ingredients of good teaching included: creating an atmosphere of mutual respect and fairness in the classroom, providing opportunities for "active learning" and humour to encourage pupil engagement, making learning interesting, and explaining things clearly.
Professor Debra Myhill, from Exeter University, took a similar line. She argued that while good subject knowledge and intellectual ability were both important, they were not "sufficient" to be a good teacher.
The crucial ingredient, she argued, was a teacher's ability to reflect on his or her own performance and then to change it.
She too argued for a healthy scepticism towards national policy initiatives.
Indeed she advocated that a good teacher should go in for "creative subversion".
By this, she meant that teachers should neither passively comply with government initiatives, nor should they point blank refuse to implement them.
Instead they should "adapt them creatively".
The third expert, Professor Mary James, from the Institute of Education drew on the massive, 10-year long teaching and learning research programme for her recipe for good teachers.
One of her top 10 requirements was that the teacher should "promote the active engagement of the learner".
Citing studies that showed the academic gains from children working collaboratively in groups, she argued: "If learners are not involved in their learning, they do not learn".
She noted that teachers liked to be given practical guidance on how to improve their teaching, yet what they really needed to develop was their own judgment of what works and what does not work in their own teaching.
This emphasis on engaging pupils and self-reflective teaching might horrify those who support a more traditional subject-based, discipline-oriented approach.
Indeed, for those with long memories, it was the politicians' loss of confidence in child-centred learning that led to the creation of the national curriculum and, with it, a system of national testing to handcuff teachers to a framework of required knowledge.
But maybe the wheel is turning?
Self-improvement is a key weapon in a teacher's armoury
The new curriculum for 11-14 year olds, due to start in September, puts much greater emphasis on teacher innovation and local adaptability to pupils' needs.
The big question now is whether - after 20 years of being told exactly what and how to teach - there are enough teachers ready to be "creatively subversive"?
Also, after years of being told in precise detail how to teach, will teachers feel ready both to devise their own way of teaching and engaging students and also constantly to evaluate and adapt their own teaching methods.
We might also ask whether it is too much to ask teachers to do this when, for some, just asserting crowd control requires all their energies.
Finally, although no-one explicitly said a "good teacher" needed to like children, I think this was implicit in their definitions.
However, Professor Myhill did say that "a teacher who hates children may be very good at class management but they are unlikely to be very good at encouraging learning".