It is sometimes said that there are two sorts of people in the world, those who have lost a parent and those yet to do so. David Cameron is in a third category, which he shares with Gordon Brown, in trying to confront, in Brown's words, "an unbearable sorrow that no parent should have to endure".
In Britain we tend not to like unapologetic ambition, and particularly in the class to which Cameron belongs, sharp elbows are not the thing at all. Cameron, though, has always been ambitious, and once that ambition found a focus in politics, he has done little to disguise it. So, he is a self-confident, ambitious politician from a privileged background, hardly a recipe for likeability. Yet there is in Cameron a human quality unusual in any walk of life but especially so in politics, where egos and testosterone can make a pretty serviceable substitute for decency and sensitivity.
Cameron is hugely gifted as a politician. He is bright, focused, fluent, charismatic and capable of ruthlessness. But his greatest gift is his sensitivity, his capacity for putting people at their ease. As a boy at school and subsequently, there were times when he was bumptious, a little too keen to be in the "in" crowd, but the example set by his parents left a huge mark. A family friend describes his mother, Mary, a model of middle-class public-spiritedness, as "one of the most compassionate people I have ever met".
But it is perhaps his father, Ian, whose influence was most marked. Ian Cameron, now 77, was born with severely deformed legs. He has had to live with, effectively, shortened shins, so that whereas the rest of his body suggested a man of about six foot two, he never grew much beyond five foot. One of his feet had three toes, the other four. Yet, at school and subsequently, Ian Cameron made light of his impediment. He was a ferociously energetic and fearless player of football, cricket and tennis. "I can do everything except ski," he once told a friend.
If it is true that most teenagers are embarrassed by their parents, how much more sensitive might a growing boy be, among cruel peers, about a parent's physical shortcomings, particularly in an age when words like "cripple" and worse were still on the verge of acceptability? You might expect a degree of embarrassed stoicism from the young David. Yet the reverse was the case.
"I don't remember Ian's disability ever being an issue," says a family friend, Giles Andreae. "It wasn't that it wasn't discussed out of good manners, it just wasn't an issue." A member of the family puts it more bluntly: "Whingeing just wasn't on the menu."
When he was 13, Cameron is said to have told a friend: "He is my role model. Dad has never let his disability hold him back. He has proved that you can do anything you want in life."
No one who meets Cameron fails to be struck by his superhuman self-confidence, his relaxed-in-his-skin affability. He knows what he is and is confident he knows what is important. Maurice Fraser, who worked with him during the 1992 election campaign, talks of Cameron's capacity for judging a situation appropriately. "There was a lot of emotional intelligence there," he says.
The birth of Ivan ("like being hit by a freight train"), it has been said a thousand times, was Cameron's first brush with real adversity, and so it was. He slept on hospital floors and saw the NHS at the closest of quarters. He and his family made life with Ivan work. They rebuilt their house for him, had two more children and got on with their lives. They did not pretend to be saints and confronted their own shortcomings with as much sunniness as the situation allowed. Asked if he felt Ivan enjoyed his life, Cameron said: "Not really. I think his life is very tough."
Indeed it was. This was not a child with disabilities who could skip happily about the kitchen in a world of his own. This was a child who could not move and needed round-the-clock care, some of it privately funded. Often his fits led to hospital visits. Last week's was preceded by a period of relatively good health for Ivan. While they knew his life would be cut short, his parents expected there would be some warning, some deterioration in his condition, so the suddenness of his death makes it all the harder.
Friends would be shocked at how totally dependent Ivan was on the love and care of those around him. And he had that in buckets. Cameron would stare into his eyes, never patronise him and keep him involved. Visitors would call the scene "beautiful", "touching" or "humbling". Dr Mando Watson, Ivan's consultant paediatrician, described his parents as "wonderful, absolutely extraordinary" for the way they doted on him, yet not at the expense of their other children. The parents of a newborn baby know the feeling of total dependency that a baby feels towards them, and the protectiveness that this engenders. He was rarely out of touching distance. Ivan grew in stature, but in no other respect. If anything, his parents' protectiveness would have grown.
Most babies can gurgle, cry, demand food. Ivan was totally immobile, at times incapable even of smiling. Yet his parents made him central to the family conversation. Unconditional love was the only sort that could keep him alive. One left-leaning professional sceptic who happened to catch the family unawares one day reported that "no child could have got more than that child got ... They just seem loving, committed, great parents".
Now that he is gone, does the guilt, the constant "did we do enoughs?", get even worse? Yesterday, Cameron seemed to confirm any parent's tendency towards self-laceration in such a circumstance when he said that after Ivan's birth, he expected "we would suffer having to care for him ... [but that] ... it was only [Ivan] that ever really suffered". Equally, will they be ambushed by an unwanted sense of release? Cameron is nothing if not grounded. Such questions may afflict him, and he must of course seek to banish them. But they are not the point. Coming to terms with losing a child is in a sense insurmountable. And for nobody is it harder than the mother.
Samantha Cameron is no politician. She likes design and retail and wants her husband to succeed only because he wants to. To most of us, Cameron is the leader of the Opposition, but to her, genuinely, his is just another job. She dislikes giving interviews, has little time for the press and loathes having to play the "political wife". She has no interest in meeting heads of state, and is likely to resist – if it comes to it – moving to Downing Street.
When Ivan was born, she was as distraught as any mother would be. Their friends agree that it was her husband's strength that maintained the couple's equilibrium. Through mutual support, there was a deepening of the Camerons' relationship, it seems, but he was, as one friend put it, the load-bearing parent. Yet throughout Ivan's life, Samantha would worry over every second of his health and care. Her job at Smythson required her to go away on business, but she never went for longer than strictly necessary, in order to get back to Ivan. David Cameron says his family comes first. For Samantha, if it is possible, that is even more true.
So how must she have felt when they returned from hospital on Wednesday morning to find the press on their doorstep? To see their awful grief on every front page on Thursday? To be fair, the photographers did not harass them, and the couple decided that press interest was unavoidable. There have been no photographers outside their constituency home and their privacy has been respected, for which, a source very close to the couple has said, they are extremely grateful. The press will stay away from the funeral on Tuesday, with just one picture likely to be distributed for press use. But Samantha Cameron did not ask to be in the public eye.
If Ivan's birth changed Cameron as a person, his death will change him still more. It would be glib, and quite possibly wrong, to say he will emerge a wiser, stronger or more rounded politician. In time, voters will form their own view of that. How it changes his wife is even more unknowable. In a sense, David and Samantha Cameron will never get over their loss. They will need every ounce of one another's love and support if they are to get close to it.
James Hanning is deputy editor of The Independent on Sunday. An updated, paperback edition of Cameron: The Rise of the New Conservative, by Francis Elliott and James Hanning, will be published next month