What makes a great teacher? It is a question that has been asked for generations, and the proliferation of titles - including advanced skills, post-threshold and excellent teachers - reflects the difficulty of finding an answer.
Now a government-commissioned committee undertaking a review of teaching standards in England has become the latest body to attempt to solve the problem.
Its working title - master teacher - may sound more akin to a medieval guild, but it reflects an ambition to uncover the magic ingredients shared by the profession's leading exponents.
The qualities of master teachers, listed in a draft document last week, are undeniably abstract.
To qualify for the title their pupils' outcomes must be "outstanding", although the document stops short of recommending a floor target. They must be analysed in the context of school, national and international standards, it insists. "I don't want the context to be used as an excuse, but I think the context should be taken into account," said Sally Coates, principal of Burlington Danes Academy in west London.
The excellent teacher status is rarely used. When it was introduced in 2006, Labour ministers predicted that 5,000 teachers would apply in its first year; in reality, just 34 did.