Powerful thing, spite. Or hatred. Or pure and total enmity. And it really needs to be because if you assess the bare facts, the odds do not look half as favourable.
There are, after all, nearly as many registered referees - 34,558 - in England as there are rugby players in Scotland.
The difference in resources is colossal. England has the most affluent union in the world, supporting the needs of more than 2.5 million registered senior players, according to the International Rugby Board website. Scotland, by contrast, has 251 clubs to support its 38,500 players.
Put another way, the statistics are similar to those separating Japanese rugby and the UAE's. And Japan beat the national team 111-0 a few months ago.
So if hatred can be used to get Scotland through, perhaps that is no bad thing.
To say that the Celtic nations loathe the English - they are not called the "Auld Enemy" for nothing - is a little bit like saying cricket is a religion in India. It is a cliche, but it has foundation because national self-esteem is so intrinsically linked to it.
"The biggest match of any Scottish rugby player's career is up against England every year in the Six Nations," Gregor Townsend, the Scotland attack coach, was quoted as saying this week.
"Now we have them in the World Cup, and for the first time ever Scotland are playing England outside their own countries."
What grates even more for those north of the border is the fact that the loathing is, for the most part, one way. The supercilious English are above all that.
"The Celts dislike the English, which is quite understandable given the history," said Mark Regan, the former England hooker, who is proud to say he never lost a match to Scotland.
"We look bigger than that in England. We look to the southern-hemisphere sides, not the Celts."
Anyway, surely such a preoccupation with one team is unhealthy?
"In the professional era, if that is the case, then England always has a head start," said Jeremy Guscott, the former England centre. "In all honesty, England should not have been the biggest name on anybody's calendar" since they won the World Cup in 2003.
"As I understand it there is a perceived arrogance about the English side from the Celtic nations. From before I played, when I played, and after I played, that has always been the case."
Guscott was a young international player when England played Scotland in two of the most vital meetings the nations have had on a rugby field.
His England side lost the first to give Scotland the grand slam in 1990, which is still celebrated passionately 21 years on.
Saturday's match in Auckland is the first time the rival nations have met on neutral territory, although they have met once before at a World Cup.
In 1991, England edged through to a final against Australia at Twickenham after winning in Edinburgh thanks, in no small part, to a missed kick by Gavin Hastings, the Scotland full-back.
"I was totally stunned," Guscott said. "Gavin used to knock kicks like that over in his sleep.
"I was stood with Rory Underwood [the England wing] talking to him about what we were going to do to get back into the game, wondering how we were going to get out of this one.
"As I was talking I looked at the kick, watched him miss, and uttered a few expletives to Rory. We said: 'That's our reprieve, let's get back into this.'
"Even thinking about it now, I can't believe it missed."
The annual Calcutta Cup matches are high-octane enough, but a World Cup meeting raises the stakes. It is likely that one of these two teams will be going home after Saturday's encounter.
"There is no bigger game and no bigger stage in a lot of guys' careers," John Barclay, the Scotland flanker, said after last week's defeat to Argentina, a result which renders Saturday's match make or break.
This is emotive stuff. Particularly in the sanitised world of the professional era. Nowadays, rugby people talk about the breakdown, the set-piece, physicality and attacking around the fringes. That must be what does it for today's drones.
It seems a far cry from the days when players were whipped into game-day mode by talk of the battles at Bannockburn and Culloden.
"I remember playing against Scotland in a Six Nations match and the boys managed to find out that the night before the game all their players went off to watch Braveheart," said Regan, who was England's hooker in the 2007 World Cup final in France. "It didn't work."
But then surely there is only so much inspiration you can derive from watching an American who grew up in Australia, Mel Gibson, play the role of a Scottish hero.
A little bit like having an Englishman in charge of your national rugby team. Andy Robinson, the former England international turned Scotland coach, acknowledged the incongruous position he is in, having to rouse his adopted side for such a charged match.
"If it means taking it out on me, with my English roots, then they've got to do that because you have to use every emotion," Robinson was quoted as saying this week.
"Sport, and rugby in particular, is an emotional game, and there is certainly a lot of history in this game."
Scottish players never need to dig too deeply to summon up animus for the English, but it sometimes can be a chore the other way round.
"I ended up playing for England because I was born here and Scotland didn't have a competitive team at the time, so when I switched codes [from rugby league] I kept playing for England," said Jason Robinson, England's try-scorer in the 2003 final.
"I was probably brought up a Scot more than anything really, though, and my mother would support Scotland in matches.
"My upbringing through a Scottish mother means I do have an affiliation for them."
Some players did not need a familial link to feel an affiliation with the side they were supposed to hate.
"My abiding memory of [matches against Scotland] is they have a very good national anthem," Regan said.
"Flower of Scotland is great and it gets the heartbeat going. Like the New Zealanders with the Haka, it lays down the challenge.
"They are laying down the gauntlet, and when you hear their anthem you rise to the challenge. Sometimes you sing along with it."
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