Some might consider the wheel, electricity or the Internet as humankind's greatest inventions.
But American urban economist Edward Glaeser says that cities are its best creation by far. Cities organise and focus vital human activities like trade, transport and teaching so well that billions have been pulled out of poverty.
But with more than half of humanity living in urban areas - and three-quarters by 2050 - how can space-starved cities house so many more people?
By building super-high apartment buildings at affordable prices, says Prof Glaeser, 46, who was in Singapore recently to attend the inaugural meeting of the international advisory panel of the Lee Kuan Yew Centre for Innovative Cities, which is part of the Singapore University of Technology and Design.
Having taller apartment blocks, says this Harvard University don, will free up more ground-level space for parks and other recreational spots to leaven the stresses of crowded lives.
All of which sounds obvious, but Prof Glaeser's crucial caveat for high-rise homes is that they must be affordable to middle-income folk. This is because such folk form the largest working group in most cities, and so are their main economic engine.
They are also the ones who struggle most to own homes, he points out, unlike the rich who have their pick of properties, and the poor, whose housing is subsidised.
He says: "It is not just about going super-high in the sky, but it is also about expecting our cities to deliver to middle-income folk a quality of life that is not much lower than that provided by the Sun Belt suburbs in the United States."
The Sun Belt comprises such American cities as Atlanta, Dallas and Phoenix, which still have swathes of land on which "unbelievably cheap" palatial mansions are built at US$70 (S$90) per sq ft and sold at about US$130 psf.
He argues that constructing towering homes that are easier on the pockets of the middle class will be most effective in keeping talent concentrated in cities, which he sees as essential to human progress, as detailed in his clear-eyed and critically lauded book, Triumph Of The City.
But why would developers in, say, Singapore be motivated to supply private, low-cost high-rise housing when they can make more building luxury homes?
He insists: "I don't think the current frothiness in the Singapore housing market will turn off the market's desire for cheaper housing."
He says that the key to making high-rise living affordable is a combination of new technologies that include vastly improved building materials and breakthrough methods of construction, including more labour-saving techniques. "It is all about building up cheaply, safely and pleasantly."
While his idea about providing affordable high-rise living in metropolises might hardly appear like a Big Idea, it has actually toppled decades of Western thinking in the other direction, that is, towards low-rise housing and suburban sprawls.
In fact, Prof Glaeser - who was made a Harvard full professor at age 31 - has given momentum to the movement known as retro-urbanism, whose leading voices include the renowned American urban theorist Richard Florida and the US Housing and Urban Development Secretary Shaun Donovan, who declared in 2010: "We've reached the limits of suburban development... People are beginning to vote with their feet and come back to central cities."
Prof Glaeser's views run contrary to those of the late American architectural critic Jane Jacobs. In her iconic book, The Death And Life Of Great American Cities, she championed as ideal residential buildings no higher than six storeys - and castigated high-rise living as dehumanising hives of social dangers.
Prof Glaeser says: "She was right to point all that out. But it's not true that Greenwich Village neighbourhoods are the only way for urbanites to survive. There isn't only one way to live."
That said, the native New Yorker who grew up in a 1,300 sq ft apartment in Manhattan, knows only too well how skyscrapers often block the best views, cast giant shadows everywhere and often become dens of moral decay. "So the important economist's caveat is that people should pay for the social costs of their choices and actions," he says. "So we should charge city dwellers for the congestion and pollution they create, and not give them infrastructure below cost and sprawling developments."
He also takes issue with Mrs Jacobs' argument that because it was cheaper to maintain old buildings than build new ones, the way to keep prices down was not to build any more.
"That's not how supply and demand works," notes Prof Glaeser, who is against preserving old buildings for nostalgia's sake. "If you have robust demand for an area and you don't allow enough new supply of homes, prices there will go up and you risk turning the area into a boutique luxury space affordable only to the wealthy."
Case in point: Greenwich Village, which only "hedge fund billionaires" can afford these days.
In all this, he insists that he is not trying to dictate that those who want to live in suburbs or large landed houses should not do so. He just believes that it makes better economic and ecological sense to pool talent in cities, where one can meet and mingle with all sorts of people. That is a great advantage in an age when those who have the freshest information fastest make the most money. After all, he notes, 80 per cent of the United States' GDP is from its three largest metropolitan areas, namely New York, Los Angeles and Chicago.
Not so fast, says his compatriot Joel Kotkin, an urban geographer who is all for suburban living. Writing in The Daily Beast in April, Mr Kotkin cites a recent Brookings Institution study which showed that jobs have been disappearing in 91 of the US' 100 biggest cities, except Washington, DC. As Mr Kotkin put it: "A funny thing happened on the way to the long-trumpeted triumph of the city - the suburbs not only survived but have begun to regain their allure as Americans have continued aspiring to single-family homes."
That, however, is not likely to happen in Singapore soon, Prof Glaeser notes: "One of the things that is different between Singapore and US cities such as Boston and New York is that there just aren't any middle-income people living in these cities because they have been priced out of it.
"But in Singapore, people don't just move to Dallas if they cannot buy property in Singapore. So they're still here and you've got to figure out a way to house them."
In fact, the widely travelled don adds, he sees Singapore's greatest challenge now as being to ensure "good qualities of life for people lower on the income ladder".
The married father of three then muses: "It's a real problem to focus on, but it's still a lot better than having unclean water, drug gangs running your streets or a malfunctioning urban school system."
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