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Paul Clarke, Chief Technical Officer at Ocado, has recently set out a vision of smart cities and communities, with tracking and modelling of flows through drones, robots and smart infrastructure, which is in this same spirit.38  Such a model is not without precedent. In 1949, the New Zealand economist A W Phillips built a machine in his landlady’s garage in Croydon using liquid and tanks to illustrate the circular flows of income and interactions in an economy (Figure 3). This early analogue computer was not for show; it was used to understand the economy and simulate policy before digital computers existed. Working versions still exist. The model I have sketched is a high resolution, digital version of the Phillips machine. 38  Clarke (2018). 28  All speeches are available online at www.bankofengland.co.uk/speeches  28  Figure 3: Phillips machine  Sources: Image available at https://en.wikipedia.org/wiki/William_Phillips_(economist)  Building a microscopic-level model of the economy is an enormous undertaking if it were to be attempted top-down. But that traditional top-down approach to model-building may itself need to be inverted. Clarke’s suggested approach is to build this model bottom-up. The map would be crowd-sourced and then stitched together, a Wikipedia-style or micro-to-macro approach to model-building. An economy-wide digital twin would, in a very literal sense, be micro-founded, but in a diametrically-opposing way to existing models. The foundations of this digital twin are constructed not on micro theory, but micro fact.