It has been dissected by the Office for Standards in Education, mulled over by academics, and whole rainforests have disappeared as researchers clamber for the truth.
But one man appears to have the answer - that there is no answer.
Many have pondered the question: "What makes a good teacher?" The reality, it seems, is that no one really knows - and certainly not one of the Government's former top advisers.
"It is a mystical, medieval craft," according to David Hopkins who, until recently, advised the Education Secretary.
Professor Hopkins told a London conference that the profession had yet to develop the language to spread the word effectively.
"Teaching has not advanced much beyond a medieval craft, and as a result the best teaching remains a dormant profession," he told slightly puzzled teachers and academics at his new workplace, London university's institute of education.
When asked to explain further it all became clear. Well, sort of.
The profession, he believes, has not yet come up with the terminology needed to describe, harness and spread best practice.
Furthermore, teachers need better access to good research to aid them in the classroom. Professor Hopkins advocates giving them time to do their own studies into which methods worked best for them and their pupils, and to share their findings.
He said: "I can go into a classroom and see an outstanding teacher and feel guilty that I am not as good as that. But we have not yet developed the language or way of working that allows teachers to take that good practice and use it to develop themselves.
"Some people will always be better than others, but they all need to have a repertoire of techniques that they can adapt to the needs of their pupils.
"The art of teaching is about diagnosing the learning needs of young people and then selecting from a range of models the most effective learning strategy."
So that's clear, then.