"Why on earth have you said 'yes'?" one of my close MP friends asked last night. Here's my answer.
Jeremy Corbyn's win was overwhelming. He was properly nominated - by us as Labour MPs - and emphatically elected by more than 420,000 members and supporters. He is our new Labour leader. End of story.
Respect for the result, means MPs need good reasons not to serve. And arguably the responsibility is greater on those of us who backed a different candidate to respond to Jeremy Corbyn's pledge to be "broad and inclusive" and a leader who "welcomes debate and wider involvement".
More telling than the voting figures was the surge of interest in Labour politics he generated during the campaign. Those who say Labour's selectorate is only a fraction of the voting public are right, but it's grudging in the extreme to suggest that Jeremy's ability to inspire thousands of people is not impressive, and important.
For four months the Labour Party has been speaking to itself. Now we must talk to the country and win public confidence and support in the dozens of seats we'll need to take from the Tories by 2020 to achieve government.
The days of simply looking to the leader and the leader's office for 'the line' are over, and with it the fiction of grown-up politicians with their own minds and distinctive views having to pretend to agree with every word the leader might have said.
This is one of the ways in which leading political figures in all parties become diminished, distrusted and dismissed by the public. Explaining and managing this with social media and the relentless 24-hour media hunt for 'splits' will be tough because people dislike divided parties. But I think the public also want plain-speaking, serious politicians and know the problems in the UK and world-wide are complex.
In areas where Jeremy's views to date may not be the same as Labour's established policy - perhaps on nuclear disarmament and membership of the EU - any change will have to be debated not dictated, in the Parliamentary Labour Party and wider Labour membership and movement. There will then need to be decisions in the shadow cabinet, national executive committee, national policy forum or annual conference.
This needn't be a weakness. Some of Labour's strongest frontbench teams have had ministers with markedly different views - from Bevan and Gaitskell under Attlee to Benn and Jenkins under Wilson or Mowlem and Cook under Blair.
However, once debate was done and policy settled collective responsibility was rightly expected and understood. This is also relevant to Labour today.
Debate but not hardening division and factionalism is vital because our paramount duty as the Official Opposition is to take our arguments to Tories, and expose the worst of what the Ministers plan to do. Our unity is urgent. Today they introduce new laws to stifle civil dissent, prevent working people having unions that can protect them and cut off funding for Labour in ways that would be totally unthinkable and unacceptable in other mature democracies. Tomorrow they will cut tax credits for millions of low and middle income working families after just 90 minutes' debate.
Having swept up such widespread support within Labour, Jeremy must now lead Labour's drive to do the same with those we have to convince and carry with us in the country. His election earns him the right to expect the support of Labour MPs in doing so.
John Healey is the Labour MP for Wentworth and Dearne, and shadow minister for housing and planning