From networks of cities to systems of cities

Philosophy then social sciences took an interest in defining what a city is and finding ways for improving its political organization, shaping its esthetics or developing its performance. Many theories flourished that provide a vision about what ‘a’ city is or how it should be. For example, economists constructed, since the first suggestions by Marshall (1890), a theory of ‘the’ city as a means of offering agglomeration economies that provides a subset of diverse externalities favorable to enhance the functioning and development of firms (Fujita et al. 1999). Among all the social sciences, geographers were the first, followed by historians and archaeologists, to engage in searching for a theory that is not that of ‘the’ city but which readily admits that cities form in networks connecting cities of differentiated size and function (Peris et al. 2018). Building a theory is a long and complex process. Ideally starting with observation, measurement, following a formalization in logical, mathematical or computing models, then validation through many empirical tests on international comparisons, theory also interferes with available data, technologies, current scientific paradigms and ideological backcloth of the time during the course of history. Many controversies and personal or political rivalries take their place in what is by no means a linear and regularly constructive process. We provide here a very simplified historical view by reducing the emergence of urban theories to three main steps: first, the observation of their relationships with territories and trade networks; second, how they integrated observation of socio-economic changes within concepts of systems dynamics and co-evolution; and third, how they may take further inspiration from the recent availability of not yet fully explored data about real-time interaction networks at all geographical scales.