Aforward makes a run for a through-ball. The defender steps up. The ball is played and the forward is in an offside position. The forward chases the ball, following it, escorting it. Something in his head – perhaps Bruno Fernandes screaming – warns him that he might be offside and so he pulls away, allowing another forward (let’s call him “Bruno Fernandes”) to sweep the ball into the net. Is that forward offside? He hasn’t touched the ball but can he be deemed to not have been interfering? Did his presence not prevent the initial defender charging back and trying to hook the ball away? It’s the sort of question that could have kept medieval theologians occupied for a lifetime. Duns Scotus and Thomas Aquinas would have written controversial tracts upon it, then those tracts would have been analysed and those analyses themselves analysed. Whole libraries would have been devoted to the theme. To what extent, if God is omniscient, can we have free will? If our thinking is flawed, can any revelation escape flaw? What is interfering? All we had at Old Trafford, though, was a brief conversation between the assistant referee Darren Cann and Stuart Attwell. It was all respectfully traditional. No use of mics and headsets, no recourse to the VAR monitor, just two middle-aged men looking worried and talking behind their hands, nodding occasionally. It could have been Ray Tinkler and Bill Troupe deciding West Brom’s Colin Suggett hadn’t been interfering and torpedoing Leeds’s title chances at Elland Road in 1971 (then as now, Arsenal were the indirect beneficiaries). Then, a final nod, and Attwell, dramatically, his habitually beleaguered features taking on a decisive aspect, pointing to the centre-circle. It’s a moment that could be critical – in the title race and in the histories of Manchester United and Manchester City. Under the law as it stands, it may have been the correct decision, but the modern offside law is a mess. Everything about Manuel Akanji’s positioning and decision-making was conditioned by what Marcus Rashford did. It may make it easier to judge whether a player is on- or offside effectively by interpreting interfering as touching the ball, but that does not mean it is right. Rashford essentially played the most protracted dummy in history. Four minutes after that equaliser, United had a second, again from a ball played behind the defensive line. A defeat that would have been seen as confirming City’s superiority despite recent form became a victory that has lifted United to within a point of them. Even if it had ended in a narrow defeat for United, that for a time seemed likely, it would have been a marked improvement on the previous three derbies; for the first time in at least a decade, they appear to be on an upward trajectory. But what of City? It says much about the standards they have set that a run of 14 games of which they have won nine can be regarded as a wobble. But the performance in the Carabao Cup defeat at Southampton was mystifyingly bad. And they had recently failed to break down Everton. That’s seven points dropped in the last five league games and, while it’s only recently that that would seem a potentially decisive blip, it’s been enough to let United close in and to give Arsenal the chance to open up an eight-point lead by winning at Tottenham on Sunday. At least temporarily, the aura, the sense of City as an inevitable, unstoppable force, has gone. That will lead, again, to questions about the impact of Erling Haaland, who mustered just 19 touches. That is who he is, and his effectiveness in front of goal – 21 Premier League goals already this season – can hardly be doubted. But he has forced a change in how City play, and not just in his lack of involvement in the long skeins of passing that have always characterised Pep Guardiola’s sides. Haaland’s directness means he needs the ball played to him quickly, to take advantage of his barrelling runs. But that has never been the Guardiola way. He has said it takes 15 passes for his side to set themselves to be able to attack with a reasonable sense of security that they will not be undone by a rapid counterattack, something to which their high defensive line, essential to the press, makes them vulnerable. Yet both goals conceded here, whatever the doubts about the first one, came from just that sort of pass behind the back four that has so often been their fatal weakness, particularly in Europe (and, yet again, a Guardiola side was undone by a flurry of goals conceded in quick succession). United are improving, the offside law has changed to seem at times absurd, but some things remain the same: when City fail, it is in ways that feel so familiar, so long-standing, that Aquinas and Scotus might have written about them.