What is the raison d’etre of a city? Is it to provide parks and cafes, theatres and libraries? Is it to offer proximity to schools and hospitals, clubs and communities, sporting arenas and shops?

If you ask Alain Bertaud, the internationally renowned urbanist we hosted at three packed lectures in Auckland, Wellington and Christchurch this week, it is none of the above. His answer is a provocation to every town planning expert. The reason we live in cities are their labour markets.

Bertaud certainly knows what he is talking about. Looking back to a career in urban planning spanning almost five decades and having advised scores of local governments in five continents, the former Principal Urban Planner of the World Bank is a globally recognised expert on urban affairs. But his sober analysis of cities may still come as a challenge to his fellow town planners.

However, as Bertaud convincingly argues, without a well-functioning labour market, no city can thrive. Detroit did not deteriorate because it lost any of the things we also associate with cities but because it lost its main industry. Without businesses offering jobs, and without people bringing their skills to a city’s labour market, there would not be the need for cities.

Labour markets construct a diverse urban community; they bring together those with complementary knowledge and skills, enable the establishment of specialised goods and services, foster innovation, and thus create the attractive cities we know and love. The larger a city’s labour market, the more productive, efficient and innovative it will be.

According to Bertaud, economics are essential to our understanding of cities. Cities cannot be engineered like buildings or bridges. They are living things shaped by the unplanned interactions of their inhabitants. Urban planners often cannot anticipate such market forces, and so cities mainly develop as a result of human action but not necessarily human design (as much as planners try to plan the unplannable).

Urban planning cannot arbitrarily dictate where firms should establish themselves or where people should live and work. It is people and businesses who have to make their own decisions – and intervention often hinders rather than helps. Urban planning needs to recalibrate its focus and accept that economic forces are the key determinant of our cities. Working with, rather than against the free flow of firms and people will then foster economic dynamism and create more efficient, prosperous and sustainable cities. This may not be what most town planners believe – but it is Alain Bertaud’s core insight having spent his entire career as one of the world’s pre-eminent town planners.