Manila, Philippines: The rich elite in cities across the world want to clear the slums which are now home to a billion people. But many of those who live in shanty towns like that which lines the banks of the San Miguel canal, do not want to leave. Why?
I had come to the Philippines to explore a theory but, as always, reality got in the way.
I was standing on the bridge over the Estero de San Miguel, a slum in the capital Manila.
My host was architect Felino Palafox and he had spread his blueprints across the parapet of the bridge and we were poring over them, with some street kids clambering around us. Palafox was making a big splash with the locals his Star Trek-style traditional Philippines shirt.
The sweep of the slum was pretty horrible - a curve of water, shacks on both sides, multicoloured plastic rubbish inches deep in the water, and now and then the sound of something hitting the water as somebody used the "wrap and throw" method of sewage disposal into the Estero itself.
I needed the bathroom myself, so somebody guided me into a shop - a kiosk really - on the bridge. I clambered down a ladder and then, suddenly, I was in a place whose existence had not really occurred to me. Because if the slum is built right up to the waterway, on stilts, how do you get through it?
The answer was a tunnel. Four feet (1.2m) wide, about 5ft 7in (170cm) high (I learned this painfully as I am 5ft 7.5in (171.4cm)) and 600m (1968ft) long.
Twelve hundred families live off that tunnel - about 6,000 people. Such is the population density that I realised immediately what the women cradling their kids and swaying absentmindedly in the half light were doing - the same as me, waiting for the toilet.
Lack of hope
When I came out I was, as Dennis Murphy said to me afterwards, "stoked". Dennis is an ex-Jesuit priest who runs an NGO in Estero de San Miguel that has helped the slum-dwellers organise themselves.
"You were hyper, manic," he told me later.
That was because whenever you enter a slum your spirits do not so much droop as plummet. A fall, with a long "aargh" such as that emitted by the Wily Coyote when The Road Runner gets him to go over a cliff.
You suddenly become aware physically - even though you have seen this stuff many times before - of that thing no modern human being wants, limitation, boundedness, a lack of hope.
After two minutes down the tunnel I stormed up the ladder and told my crew to stop filming Palafox. Nice though his scheme development plan was, it was on paper. Down in the tunnel was a reality that, despite being in Manila's slums for days, we had not properly seen.
Mena Cinco, the barangay captain - a kind of local councillor with the authority of a tribal chieftain - led me down again.
We met Rotsi and her family - mum, dad "a driver for a Chinese family", an unspecified family guest, a daughter doing her homework and a toddler. Five people in one-and-a-half rooms.
"We've been here 20 years," Rotsi told me.
Next door Oliver Balderas was snoozing with his kids, who were eating ice cream. There was a cartoon on the television and mum was also having a nap - it was about 32C and heavy with humidity.
They came to the door. Mr Balderas is a construction worker earning about $3.50 (£2.13) a day. The family moved to Estero de San Miguel from a conflict area 10 years ago.
The room - about 8ft (2.4m) square, and like all of the Estero, built of wood and floored with lino - is their entire dwelling space.
Manila is undergoing a population explosion. Of the around 60 people-an-hour estimated to be arriving here, about half are coming as migrants from the collapsing agriculture sector, and half are born here - so there are kids everywhere.
These kids sing a song about the inevitability of poverty and their determination to overcome it.
With the sky glowering when I got out of the tunnel, I was no longer in any mood to go on giving the theory the benefit of the doubt.
My instant reaction was this: "There's a theory that says basically slums are here to stay, that they're cohesive, sustainable - green even.
"I can see the social cohesion bit, but as for green, well, (my nostrils flare at the river smell).
"And I can't help thinking the whole theory is a bit of a cop out because why - when in the 19th Century they cleared out places like this in one generation do we, in an era of globalisation, tolerate them?"
If I came out of the Estero de San Miguel "stoked", it was because it challenged my trendy notions, learned from the 2003 UN Habitat report and interviews with various experts, and re-awakened the inner Edwardian-era social reform nostrums my grandparents taught me about slums, which is that they have to be cleared.
But then I went back into the San Miguel by night, with Mena still trying to educate me about the social cohesion, and I was forced to rethink it all again.
I met business graduates, found an internet cafe, met the volunteer police force and got offered the chance to eat a boiled egg with a chicken embryo. I said I would rather jump in the canal naked, and the local women invite me to do just that.
Then, over a beer with ex-Father Dennis, discussing our mutual experiences with the Salesians and the Jesuits, I discovered what one billion people on the planet have discovered - slums are not so bad.
They have changed from the Dickensian hell holes of our imagination. Through education and communications technology people are making life bearable for themselves - and of course providing the modern mega-city with an indispensable workforce of cheap labour.
The result is we have to confront a question that would have appalled the 19th Century pioneers of city design - do we have to live with slums forever?
I do not know the answer to that question - but I now understand the question.
Somewhere between the theories of the architects and NGOs and the rigid clearance doctrines of Prada-clad Filipino millionaires, and the night on the streets with the local cops and the day in the countryside with people whose main ambition in life is to live in a Manila slum… I have gone beyond the theory and experienced the reality.
Join Paul Mason on Tuesday 16 August at 20:00 BST on BBC Radio Four, on Newsnight at 22:30BST on BBC Two, and on BBC World TV's Our World slot on 26/27 August, and on the BBC World Service's One Planet on 19 and 26 August to experience it for yourself.
And for more background read Paul's New Statesman article here.