When , uniting the country and beating Corbyn, it is very much a personal imperative. For the simple reason that if he fails, as many of his colleagues anticipate, he will look like a blithering idiot.
The point is that back in the spring of 2016, he faced a nation-defining fork in the road, when he was dithering about whether to stick with the then PM and fight to stay in the EU or lead the leave campaign.
Cameron will believe till his last breath that Johnson’s defection to Brexit tipped the scales against him. And when Cameron lost, May took over, and the rest was chaos and mayhem for the Tory party.
But let’s play the game of counterfactuals.
If Johnson had stuck with Cameron and Remain had won, Cameron would have stood down as PM roughly when May did, this spring. You will remember Cameron pledged to quit then in 2015.
So in the last few weeks we would have seen Johnson jousting to be Tory leader - probably against George Osborne - in wholly different political and national conditions: a Tory party and nation probably less fractious and divided; an economy not suffering the leak of capital abroad nor the fear of a Brexit-induced dramatic worsening in the terms of trade.
Who knows whether Johnson would have beaten Osborne in those circumstances. But most senior Tories reckon he would have succeeded Cameron.
So Johnson would have inherited stability and relative success, rather than the gravest crisis this nation has faced arguably since 1940 - a crisis for which he has his share of responsibility.
So having achieved his lifelong dream of becoming PM, and by the most tortuous and painful route, it is a matter of honour for him that he doesn’t totally mess it up. Oh, and it sort of matters to the country too.