Those who have lost a child say that the depth of grief is impossible to imagine. No less so for parents who live with the knowledge that their severely disabled child's life would be short.
Imagine then having to cope with that grief in the public eye. The life, and now sad death, of Ivan, David and Samantha Cameron's first child, will shape the man who may be our next prime minister.
Ivan was so severely disabled that he could neither walk nor talk, and for a long time the drugs he took made him unresponsive to the love that his parents lavished on him. Friends have told me of the joy David Cameron felt when, after a change of medical regime, the Tory leader could declare: "Ivan has got his smile back."
Mr Cameron has described discovering the scale of his son's disabilities as like "being hit by a freight train". Non-political friends of his have told me that it changed a man who, until then, had had a privileged, happy, and in many ways extraordinarily lucky life.
Ivan's regular medical crises forced many emergency trips to hospital - nights slept on the floor for his parents and bonds formed with people from backgrounds very different to the Camerons.
I recall receiving a call from Mr Cameron apologising for cancelling an interview we were due to do at the end of his holiday. Ivan had been taken ill and he was now driving his son miles to the nearest regional hospital for treatment.
Today I have no doubt that politics will be suspended, not just because it is the right thing to do, but because Gordon Brown I suspect will be reliving the moment his first child Jennifer died. It is a tragic bond that connects the prime minister and the man who would be prime minister, who are different in so many other ways.