9 Rethinking the relationships between the city’s infrastructure, the flow and expression of people is questioning human habitat and human identity itself. Does the answer lie in buildings? It depends on how they’re defined in the 21st century. Perhaps the answer is in the concept of sustainability: if something is durable, it has no attachment (or has flexible attachments) to fixed references, like so it directly addresses needs rather than expectations. It is also about recognising that the challenge concerns all individuals in an urban community, because human welfare is a concept linked to society. Whether the city is designed by architects or not, human welfare will be affect. To rebuild the idea of a city would be to think of a support system for society rather than a blunt reflexion of its realties. The idea is to build cities around social movements and networks, which changes radically the way architects intervene. Financially profitable versus economically feasible, another debate that also requires the understanding of sustainability. Public opinion puts financial profitability first because it is automatically linked to unlimited choices, neglecting the idea of economical human welfare which is the sum or balance of different variables answering to a certain need. The struggle to preserve the environment is a consequence of that: in the search of increasing choices for consumption, the balance in the link between nature and human welfare was lost. Going towards sustainability we redefine growth according to contributions made between human beings and nature for both human welfare and conservation of ecosystem services. A key parameter in this challenge would be information exchange in that for example we are now aware of the Jevons paradox concerning energy efficiency leading to possibilities of greater consumption. Social infrastructure is at the heart of this new growth and the architect is an observer that suggests. It is about the ability to propose development paths and expand information flow, so that the city is formed by a multitude of inputs.