Nothing prepared me for the feelings I experienced when my first child was born. It was as if I had discovered a new way to love. A feeling that I previously never knew existed. I had been quite nervous about how I would react. I had never been a fan of babies. But the surge of emotion that welled up from within as she emerged into the wider world banished any doubts I might have had.
She was with us at the hospital when we went for the twenty week scan for our second child. At just two and a half, she was bubbling with excitement about having a sibling, and I don't think anything would have stopped her from coming with us to get a first look, however fuzzy, at "the baby in Mummy's tummy".
But as the stenographer stared at the screen more and more intently, the realisation grew that this wasn't going to be a routine scan. Various doctors came and went, until finally they invited us into a side room where they told us our child had Spina Bifida.
Of course, the instant reaction as a Dad is to try and fix things - even when like me you had no idea about the diagnosis you have just been given, let alone the prognosis. "Well, surely there is something that can be done?" I asked. "Yes, there is," came the instant reply. "You can have an abortion."
And they left the two parents and their two-year-old daughter alone in the room to make a decision about whether their family would still be four.
I learned quickly afterwards, quite how blinkered my view was. It was of course, not my son that needed "fixing". It was the world around us. Just as having my first daughter opened up an entirely new window on the world, so did having a son with a disability.
The love of course is no less. But the battles are so much more. I have encountered a world that I previously wasn't really aware of. A world of barriers in which you have to battle for everything - from a place at your local school to an operation on the NHS, right through to a space on a bus or a table in a cafe. A world where only a fraction of what is supposed to be a public transport system is truly public. A world where your son can't pop around to play with friends in the holidays, because the house isn't accessible. A world geared up to the values and needs to the non-disabled.
Yes, you put aside your fantasy of having a son who plays football for England. But of course 99.9% of us do anyway, and that's really incidental. The stakes are a hell of a lot higher.
I guess a dad can go one of two ways in such circumstances. They can get demoralised, lose the will to fight and just give in. Or they can find the strength to fight. I am not sure quite why I have fallen into the latter category. Looking back, it could easily have been the former. But I appear to have become a fighter.
It is perhaps because I have seen how even just the presence of my son is changing the world for the better.
We had a two-year battle to get my son into our local primary school. It was in fact a church school at which I had been a governor. I had to resign in order to fight the tribunal. And it split the local church, where I took the Sunday School. To this day we are still discovering people we thought were friends, who behind our backs had secretly opposed Samuel's inclusion.
But we won. And the changes began even at Samuel's first sports day. They had lined him up to "run" the hundred metres. As the only child in his class with a wheelchair, I don't think they had really thought it through. And about 15 seconds after the starting gun was fired, it was clear they hadn't.
As the penultimate child crossed the finishing line, there was Samuel, half way down the track, pushing the joystick on his little powered chair as far forward as it would go. The cheers subsided to silence. But then someone in the crowd began to chant his name: "Samuel, Samuel...". Others joined in, until the whole crowd cheered him across the finishing line.
As parents we always tell our children it's the taking part that matters, not the winning. Deep down inside, of course, we are secretly very proud when our children do win - whether it be a race or an A* in our school system characterised by a culture of testing and league tables. But I can honestly say that at that point, in that school, every parent, teacher and child really knew that it was the taking part that really mattered.
Being the father of a child with a disability has profoundly challenged me. But it has also profoundly changed me. It has even led to being the first leader of a political party in the UK to job share, with our MP Caroline Lucas, so I can continue to support, love, and learn from, my son.
It has changed my idea of being a father. But also what it means to love. How we define success. My values. My outlook. And with my hand on my heart, I am convinced I am the richer for it.
Jonathan Bartley is the co-leader of the Green Party
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