Keynes Lecture in Economics, delivered by Professor Mary S Morgan FBA, on 16 October 2013 (venue: The British Academy). What would have happened to economic growth if the railway had not been invented? What if the monetary system behaved like a hydraulic machine? And why have economists argued about the exchanges that Robinson Crusoe might have made with Friday on their desert island? Professor Morgan uses materials from the history of economics to illuminate how economists have created and used models - devices that combine elements of the factual and fictional - to explore the economic world.