Though an inquest might provide some of the answers to questions people are asking today, the truth is that none of us will ever know, even if he left a note, what was going through Robin Williams’ mind in the final moments of his enriched and enriching life.

No two people are the same. No two people’s depressions are the same. But his death shows once more that depression is no respecter of class, race, profession, wealth or talent. 

His wife, Susan Schneider, said today: ‘As he is remembered, it is our hope that the focus will not be on Robin’s death, but on the countless moments of joy and laughter he gave to millions.’

It is certainly easy to recall moments of joy and laughter. Every parent can name films that they were made to watch again and again because their children loved to watch them again and again, and Mrs Doubtfire was in that category in our house.

I find it harder, however, not to focus to some extent on his death. We live in a celebrity culture, like it or not. It means that around the world in coming days, people will be talking about this terrible event, reflecting not just on his career, but on the reality of the illness which led him to his death. He will forever be remembered for his acting and comedic skills, and the films are there for all time for future generations to enjoy. But I hope too that he will be remembered as someone whose death was another staging post in the changing attitudes to mental health and mental illness which are desperately required all around the world.

If he had had a heart attack, if he had lost a long fight with cancer, if he had been knocked over by a car, would there be a need for a debate about ‘what this says about the state of heart disease,  or cancer care or road safety’? Possibly, but I doubt it. There still needs to be debate about depression as an illness, because there is still a lack of understanding that illness is exactly what it is.

Almost exactly a year ago today, in the same room where I am sitting now, I said to my partner Fiona that I honestly couldn’t see the point of living. It was not the first time I had said it, nor the first time she had heard it. What it meant was that a bout of depression, which I had felt coming on for a few days, had reached its full grisly impact, rendering me physically weak, psychologically drained and emotionally desperate. Fiona had seen it often enough – at least once or twice, sometimes more, in most of the 35 years we have been together – to know it was unlikely I would take that thought to its logical conclusion. Nor do I believe I ever would, though whenever someone does, like Robin Williams, it makes you wonder how many times he survived suicidal thoughts before finally succumbing.

I am lucky to have a supportive and understanding family, a small number of good friends, and a doctor I trust. When finally we spoke, he put me on anti-depressants, and I have been on them ever since, which is the longest period I have ever been on them. The many people I have met since, over the past twelve months, would have no idea, and it is not something I would broadcast unless asked – I t0ok a decision after my breakdown in 1986 always to be open.

Now it may be that I could have come off the medication at some point in the last few months. Between us, my psychiatrist and I decided against. I don’t like being on medication, but it is better than being depressed. And the last time, when I came off after three months, I fell back into a bad depression almost immediately.

So I have been taking pills to minimise the impact of feelings that lead someone who most of the time has a fairly positive view of the world, who loves laughing and making people laugh, thinking and making people think, to consider that life is not really worth living.

Openness has been a great help to me personally, I know that. I don’t know if Robin Williams was open with others about his depression. But I do know people in many walks of life who find it impossible to be so.

One of the things about being open about having mental health problems, and being an Ambassador for the Time to Change campaign, is that complete strangers talk to you about their own mental health problems. At a recent meeting in Westminster, I was really moved when Tory MP Tracey Crouch said she had decided to be open about her own depressions after hearing me talk about mine on Radio Kent. Another man at the meeting said he had first been open after hearing me speak about depression in Cornwall. Then Alan Howarth, Tory-Labour defector, ex MP now peer, stood up and said he wanted to open up about his depression too.

That is partly what the Time to Change campaign is all about, joining up those dots so that everyone feels they can be open. Because here is the other side of the story: a woman, an NHS nurse no less, a depressive, who said that the last time the dreaded illness hit her, and she was unable to get up, the elder of her two daughters came to see her in her bedroom. ‘I asked her to phone the hospital and say that my younger daughter was off sick and I had to stay and look after her,’ she told me. ‘Then I had to tell my younger girl she couldn’t go to school in case any of the other hospital staff saw her when picking up their own kids, and my lie to her boss would be exposed.’

Just think through all that. In Britain, 2014, one of the most advanced countries in the world with one of the best health services and among the most liberal attitudes, an NHS nurse felt she could not tell the truth about her illness. Worse, she had to draw both of her daughters into the lie, and keep one of them from school as a result.

What would she have said if she had flu? That she had flu. What if she had a cancer scare and had to go for tests? Oh that’s fine. We all understand cancer, it is a ‘proper illness.’

Here is where it is important to take heart from other campaigns. Cancer used to be ‘the big C.’ We didn’t like to talk about it. Now we do talk about it, and that is part of the reason why funding has improved, the treatments have improved, campaigns have been strong and successful.

Mental health remains the Cinderella service in healthcare because it remains misunderstood, and it remains the one nobody wants to talk about.

I can hardly imagine the horror that Susan Schneider is going through right now. It must all seem so senseless. I will join all those who try to remember her husband as a great actor, who could make us laugh and make us cry, and make us think. I will certainly remember, and continue to mimic with my daughter, his Scottish accent in Mrs Doutbfire.

But I hope that his family, those he leaves behind today and those as yet unborn, can one day look back and say not only was he a fine entertainer, but also that his fame, and the debate about depression sparked by his death, led to greater understanding, which meant that others were able to be more open, ask for help when they needed it, know that treatment was there because the politicians and health policy makers knew that it had to be.

We have a long long way to go. I met Health Secretary Jeremy Hunt last year. He wanted to talk about the campaign work I was doing on alcohol abuse. But we also talked about depression and he said that he found it really hard to understand ‘why someone like you, with the life you have, would have depression.’ I was, I admit, quite shocked. But he was relatively new in the job, and I did not explode as my daughter later told me I should have done. He was reflecting an opinion that many hold, and this is the fight we have to win – to understand that depression has nothing to do with who you are, how much you earn, how popular or famous, unpopular or unknown, you are. It just is. Like cancer is. Like asthma is. Like diabetes is. Some people get it, some people don’t. It is an illness, a truly horrible illness, and must be viewed and treated as such.

The most retweeted tweet I have ever done was one I posted when Stephen Fry had talked of another suicide attempt. ‘To those asking what Stephen has to be depressed about,’ I asked ‘would they ask what he has to be cancerous, asthmatic or diabetic about?’

To those tempted today to ask what Robin Williams had to be depressed about, with his success, fame, wealth, lovely home, lovely family, tons of opportunities and great moments in his life, please just stop. If he had died of cancer, would you ask what he had had to be cancerous about? If he had been run over by a car, would you have asked why he went and got himself run over?

Time to Change is a campaign whose time has come. Because it is time to change attitudes to depression and other mental illnesses that frankly belong in the dark ages. One day, we will look back and wonder how on earth we used to believe that depression was a lifestyle choice, only to be debated and taken seriously when an A List film star took his life, and the world filled with people saying how shocked and saddened they were.