Paul Ackford, the former England lock, once said that the essence of an international coach's job is making the right decisions in selection.
The rest, he continued, "is largely self-serving hokum." And if one selection decision towers above all others in importance, it is getting the right bloke in the pivotal position of fly-half.
Time was when it was easy for Scotland coaches. In quick succession, the country produced Ian McGeechan, John Rutherford, Craig Chalmers and Gregor Townsend. There were debates to be had about how their talents could be used - both McGeechan and Townsend played a number of their games in the centre as well - but the fact that all four became Test Lions speaks volumes for their underlying abilities.
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Things have been a little trickier since those glory days. Dan Parks won 67 caps for Scotland, but it would be understating things to say that he struggled to gain the unqualified love and support of the Scottish rugby public. With the possible exception of his magnificent 2010 Six Nations season, there was always a feeling with Parks that he was simply holding the fly-half fort until someone better came along.
Problem is, no-one ever did. Over the past few seasons, the position has been occupied by Ruaridh Jackson, Duncan Weir and Greig Laidlaw, but there has still been a makeshift feeling about the arrangement. Wearing the jersey is not the same as owning it. It is hard to recall a moment when any one of those players stood head and shoulders above the rest. Laidlaw's return to scrum-half ought to have simplified matters, but still the arguments go on.
And now the debate has been given a new twist. Greig Tonks' days as a professional playmaker may have begun barely a month ago - he was moved from full-back to 10 when Harry Leonard aggravated a thigh strain on the morning of Edinburgh's Heineken Cup match with Gloucester at Kingsholm - but he more comfortably than anyone could have predicted. He was the guiding influence as Edinburgh snatched their sensational win against Gloucester that day, and subsequent victories over Leinster and Perpignan also testified to his composure and all-round abilities.
Sunday's gruesome 38-6 defeat by Munster at Thomond Park was a setback only on paper, for in the ground itself it looked like Edinburgh's problems lay elsewhere. Given that Duncan Weir may well have played himself out of the reckoning for Scotland's opening Six Nations match - against Ireland in 12 days' time - in his error-strewn cameo as a replacement for Glasgow against Toulon on Saturday, Tonks has certainly given Scott Johnson food for thought.
"I like the versatility," said the Scotland coach of Tonks last week. "I do like that. It is a nice option to have and he has done really well at 10. I think he has shown great resolve defensively and is a smart rugby player. He is certainly making us ask questions and he is putting pressure on the others."
It is sometimes forgotten that the former Edinburgh coach Michael Bradley had plans to give Tonks a run at fly-half when he brought him to the capital at the start of last season. For various reasons, that never happened on Bradley's watch, but Tonks has displayed an improbable level of assurance since he was handed the jersey last month.
Johnson might hesitate to give the 24-year-old quite so strong an endorsement as Scotland head to Dublin, but the coach's reference to versatility suggests the posibility of a bench place.
Would Tonks back himself? You could call it a trick question, for a fly-half who does not have that level of confidence isn't really a fly-half at all, just a bloke who happens to be there. "I would be up for it," Tonks smiled. "I would love the opportunity to do it but it is not my decision, is it? I have quite a few games under my belt at 10 for Edinburgh now, and they have not exactly been easy games at a low level. I like to think I can compete at that level in the position."
In person, there is nothing arrogant about Tonks' bearing, but that calm self-belief is the very least you would want of a player who is being asked to run the show at Test level.
Capped at full-back on Scotland's summer tour to South Africa, is he surprised to find himself in the frame to be an international fly-half now? Just a bit. "I knew from the start that I can play 10 and I will play as well as I can and see what happens when the chance comes," he explained. "But it certainly wasn't expected a month ago . . . I would probably be comfortable going into that position now."
n The Edinburgh scrum-half Sean Kennedy has joined London Irish on a month-long loan.