Fear. It comes in many guises: fear of the unknown; fear of failure; fear of success. In the second round of the Six Nations every facet of that concept was apparent – but manifest in hugely different ways.
The fear of a humiliating home defeat against a newly confident Italy drew one of Scotland's best displays in years and the individual contribution of full-back Stuart Hogg enhanced his Lions credential no end.
Fear of failure accosted the protagonists in Paris in a vastly different way. Two sides who we know possess significant attacking talent had their ambition crippled to the point where they looked for the familiar and sought contact rather than endure the risk that accompanies a foray into space or an offload in the tackle.
The stultifying effect of this safety-at-all-costs attitude reduced the contest to such poor fare that people were moved to consult their TV schedules to see if 'Splash!' is still on as a potential distraction.
Reversing a run of eight successive defeats was imperative for Wales and they and their supporters will rightly not care about the entertainment rating of a game which is now behind them.
You could tell by the fact that no less a player than Brian O'Driscoll took his eyes of the ball and spilled a straightforward pass in his own 22, that fear also infiltrated Ireland's game against England. Although the margin of England's win does not appear large, it was the way in which it was achieved that should satisfy coach Stuart Lancaster.
It is another test passed; another brick laid. In the cauldron of the Aviva Stadium, a lesson in discipline was learned by this young, but rapidly maturing side.
The fact that England did not give Ireland a single shot at goal in the first half was a testament to their tremendously disciplined defence.
It showed decent line speed and effective first-up tackling in the main and when breached was able to scramble effectively and deny Ireland any real chance of crossing their line.
This replicated the only positive to come from the autumn loss against Australia and it showed this England side's ability to absorb and put into practice lessons learned in defeat.
What they should be able to take from yesterday's win is the knowledge that, provided they keep their shape and discipline around the contact area, they are capable of resisting fierce pressure and playing their way clinically into scoring positions, then making opponents pay for inevitable transgression.
England could have folded in the second half to the pressure applied by substitute Ronan O'Gara, a resurgent scrum and a fierce driving maul. The crowd were energised for the first time in the game and fragility under pressure started to see England lose the discipline which had served them so well – and the departure of James Haskell should have seen Ireland drive into the lead.
In fact, the period without Haskell was by far England's most impressive and the fear that had begun to assail them was eschewed in favour of a limited, but focused, succession of line-out drives and drives round the corner, which gave the half-backs the room to kick deep into Ireland's 22.
Man of the match Chris Robshaw took the final line-out throw of the game and is developing a case for leading the Lions to Australia.
England are developing a case for being hailed as a very good side.
This could all be rent asunder by France or Wales in weeks to come – but at the moment it looks good.