Safety first by May? Not so much
Here's the theory.
Gavin Williamson is a smart operator, a talented politician, who has proved himself loyal to Theresa May by running her leadership campaign and then getting through the nightmare of holding the Tories together with no majority, so far.
This very fragile government has not lost a vote on its own business.
Theresa May's programme has been much curtailed by the political reality. But she has not, so far, been humiliated in Parliament in the way that, the morning after the election, it seemed quite feasible that she would be.
- Williamson is named defence secretary
- Profile: Who is Gavin Williamson?
- Fallon resigns saying conduct 'fell short'
The restive right have been held back from making significant attacks. And ardent Remainers have been handled carefully enough not to blow up (so far). That is a kind of achievement, and it is in large part down to the capabilities of Gavin Williamson. So why not reward him?
Secondly, the prime minister also wants to promote the next generation in the Tory party, to give the impression they aren't simply a busted flush. Promoting one of their number is a move in that direction. And Williamson is not from the Tory Home Counties either.
And beyond keeping a tarantula as a pet - pictured here by his successor...
...and suggesting that he is a fan of the "sharpened carrot", rather than the stick, there is not much that Gavin Williamson has said or done in his previous political career that is in the public domain that means any embarrassments or problems will be hung around his neck in his new job. (So far at least).
For all those reasons therefore, it is good logic to allocate the former chief whip, Yorkshireman and Staffordshire MP this hefty promotion. Sources within the MoD say it's a good appointment because he is regarded as a very good politician who has shown that "he can get things done".
Here's the other theory though - the decision isn't smart, it's hugely risky.
Problem one, Gavin Williamson has never worked in a government department, he's never been a minister before. Undoubtedly clever, but moving him into such a huge government job straight away is a gamble.
As the Tory MP Sarah Wollaston suggested in a gently cutting way, "there are times when offered a job that it would be better to advise that another would be more experienced and suited to the role".
A former government insider who knows him well suggested the move shows Number 10's judgement is "whacko".
Second, when all the political parties are in the grip of allegations of sexual misdemeanours and trying to keep a lid on - shall we say - personnel issues, is it really a good time to be moving the man who is charge of party discipline?
And third, while promotions are always going to make some people unhappy, some Tory MPs are furious, believing that Mr Williamson manoeuvred himself into the job, playing on the prime minister's vulnerability.
One minister told me it was "appalling": "She is so weak she has let Gavin Williamson appoint himself".
Another MP said: "She is too weak and overwhelmed to spot his scheming".
A senior Tory said: "MPs are deeply unhappy he has used the position of chief whip to benefit himself and has deserted his post at such a crucial time".
'Bunch of boys'
There is no shortage of critics of the appointment, a former minister told me it was "outrageous - we are in the grip of a bunch of boys, when we need serious big beasts leading us. Defence needs someone who is able to fill at least one of Fallon's shoes".
And while Mr Williamson would deny or laugh off any suggestion that he has leadership ambition, others in the Tory party see this move (perhaps inevitably) as part of his attempt to build a bigger power base for a run at the leadership after Theresa May.
Gavin Underwood doesn't have quite the same ring as Frank, but jokes and conspiracy theories are already doing the rounds about his secret plans for world domination, tracing the fictional footsteps of the main character in the American version of House of Cards. (Take with at least a sprinkle, if not a large pinch of salt.)
Despite all the talk of Gavin Williamson's loyalty, this is not a safety first announcement.
The prime minister could have moved other ministers from the Ministry of Defence upwards. But for all the calculations today about whether it is a smart move or something she will come to regret, it is time to see what the new defence secretary is made of.
He has learnt as chief whip that being effective is not the same as being popular. That may well come in handy.
And here's the irony, the man who was meant to make sure that Tory MPs behaved themselves has found himself a rather good new job - because one of them did not.