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As Descartes pointed out that the reasoning is an interpretive process dealing with complexity analytically and synthetically, proper scientific theories of cities are needed for our rapidly changing cities. However as I mentioned, the validity of scientific theory only exists within a certain time and space, and often evolves with the increasing complexity. Therefore urban scientists need to be extremely cautious and sensitive when creating such a ‘universal theory of cities’ and understand its validity regarding time, scope, scale and context in each individual research question. Before creating a ‘theory of cities’, it is essential to understand urban dynamic complexity and its driving forces. The nature of cities as agglomerations of human settlement defines three elements: human, nature, and settlement. These three elements serve as building blocks of urban complexity, intertwining with each other structurally or organically. The challenge is that we are not only facing rapidly increasing complexity in each individual element (for example, human genome, astrophysics and communication technology), but more importantly, a geometric growth of urban complexity in the age of big data. Therefore there will be some universal theories of cities, but they would be replaced fairly quickly by the new ones because the fast growth of complexity.
Furthermore, it is difficult to adopt a universal theory of cities because every city is different. There is no city built purely based on a utilitarianism principle, because as the vessel of human settlement and symbol of prosperity, city is also often shaped by history, culture and philosophical views of human-nature relationship and human society. The cultural difference still highly impact people’s settlement preference, social networks and life style. Historical factors also could have an influence on city development such as racial and social segregation in USA. Therefore, urban science would not be universal since cities were not built purely on scientific reasoning.
So would it be worth spending time and resource to develop such a science of cities? The answer is affirmative and we need learn from other established paradigms such as biology and ecology to better understand cities. It is necessary to study cities beyond empirical observation and determinism composition to truly understand the mechanism of urban system. Just like biologists studying animals’ digestive system, it requires urban scientist look though the skin and feature of cities to see how they consume energy, its metabolism and how it adapts to climate and survive from disaster. The universal theory of urban equilibrium is valuable because many cities are facing universal issues such as energy consumption, water shortage, environmental degradation and disaster resilience. Beyond study each individual cities, we need to consider cities as different ecosystems to understand urban dynamic regarding allocation and competition. Moreover, the fact that city as a human agglomeration means there will be potential organizational benefit such as innovation and collaboration which beyond equilibrium and adaption.
City is a relatively recent product of human civilization and in last century human society experienced a mix of architectural determinism and social administration in cities, which brought us modern urban prosperity but also long-term social and environmental issues. Development of information technology and data science brings us to a new era that cities are increasingly mediated by digital technology. The large amount of city data enables us to study city with more scientific approaches to experiment and analyze urban phenomenon. New research topics such as social physics, collective intelligence and data driven society are emerging.  More importantly, data and science are no longer kept in labs but exist in everyone’s daily life, and people are engaging with city more actively and spontaneously. With careful calibration and proper approaches, this transition will eventually enable us to have a more responsive city and collaborative society.
 Ali Madanipour, Designing the City of Reason, Foundations and Frameworks (Routledge, 2007), 3
 Michael Batty, The New Science of Cities (The MIT Press, 2013), 25-27
 “The Surprising Math of Cities and Corporations,” TED Talk by Geoffrey West in 2011, accessed Sept. 21, 2015, http://www.ted.com/talks/geoffrey_west_the_surprising_math_of_cities_and_corporations?language=en
 Kingsley Davis, The Origin and Growth of Urbanization in the World, American Journal of Sociology,1995, 60:5, 429-437
 Arup and Royal Institute of British Architects, Designing with data: Shaping our future cities, 2015. https://www.architecture.com/Files/RIBAHoldings/PolicyAndInternationalRelations/Policy/Designingwithdata/Designingwithdatashapingourfuturecities.pdf
 Alex Pentland, Social Physics, How Good Ideas Spread- The Lessons from a New Science (The Penguin Press, 2014)
The concept of “urban science” emerges from the rapid development of information technology and its applications in an urban context, yet with terminological ambiguity and various interpretations. There are three essential notions to fully understand the term ‘urban science’ and its ambiguity. First, the notion of ‘science’ defines conceptual scope in this discussion. Etymologically, the term originated from the Latin word ‘sciens’ and ‘scientia’ which mean ‘to know, to understand’ and ‘knowledge, intelligence’. Historically the notion of science started as a branch of philosophy, especially, philosophy of nature, and later developed as a disciplined way to study the natural world. Therefore science is a philosophical driven and disciplined approach to better know the world. Scientific theory is based upon ‘an asymmetry between verifiability and falsifiability, which creates logical forms of universal statements’. It provides universal knowledge and the law of the nature but within a certain time and space.
Second, it is essential to understand the definition of a new science and its emergence. Epistemologically a new science defines a new paradigm, which contains distinct set of concepts, theories, methods and standards. Scientific progress and revolution are typically based upon previous paradigms, the division or recombination of mature paradigms and technological innovations. Thus if there is such a new science of city, we need to clarify what are these founded paradigms urban science bases upon. Applied science as a discipline of science focuses on the extensive knowledge development of pure science and practical applications in the real world practice. Applied science such as engineering and healthcare connects nature with human and converts theoretical knowledge to productive applications, and mostly contributes to the prosperity and development of our modern society. On the other hand, inspired by natural science and social concerns, the origin of social science started in the Age of Enlightenment around 1650s when scholars began studying human society in a systematic way by using proper measurement from formal sciences such as mathematics and statistics along with empirical approaches. In late 20th Century, social scientists adopted certain scientific research methods to conduct experiment and test philosophical theories (for example, Bobo doll experiment to test social learning theory).
Finally, the notion of the uniqueness of urban science calibrates urban scientists’ research and practice between natural science, applied science and social science. The fact that cities as agglomerations of human habitat constantly shaped by both nature (natural disasters and climate change) and human (economy, culture, innovation) defines cities as a combination of science and humanity.  Therefore urban scientists need to frequently maneuver themselves between the dispassionate objectivity, social concerns and innovative applications. In short, if there is such a science of cities, it has to integrate previous paradigms such as scientific theory, social theory and urban theory systematically.
 David C. Lindberg, The beginning of Western science: the European Scientific tradition in philosophical, religious, and institutional context (Univ. of Chicago Press, 2007)
 Karl Popper, The Logic of Scientific Discovery (Routledge, 1959), 19
 Thomas S. Kuhn, The Structure of Scientific Revolutions (Univ. of Chicago Press, 1962), 15.
 Alvin Gouldner, “Explorations in Applied Social Science,” Sociological Practice 5(1989) 7:1 26-42
 Adam Kuper and Jessica Kuper, The Social Science Encyclopedia (Routledge, 1985)
 Henry Sims Jr. and Charles Manz, Social Learning Theory, Journal of Organizational Behavior Management, 1982, 3:4, 55-63
 Mark Gottdiener and Leslie Budd, Key Concepts in Urban Studies (SAGE, 2005), 4-5.
Started from March 2015, the City of Boston initiated a crowdsourcing city project to redesign the City Hall Plaza in North End Boston. Although with a perfect proximity to most notable attractions in downtown, the current plaza is an eyesore with 20,000 square feet of vacant brick which causes a huge waste of real-estate and public space. Using the hashtag #CityHallPlaza, the mayor’s office started crowdsourcing ideas for this project.
Boston has been actively seeking urban solutions through general public by various type of crowdsourcing focusing on various issues such as snow plowing and city operation. The Martin J. Walsh established Office of New Urban Mechanics, which is a civic innovation group under the mayor’s administration to form partnerships with residents, educators and entrepreneurs for identifying city problem and seeking potential solutions.  In December 2014, the city announced an app called Permit Finder co-designed by public, technology firms and city employees to let citizens track their permit applications by phone or tablet. The idea originated from a two-day hackathon hosted by the city to design a better licensing tracking system and code a user-friendly interface. The app officially launched on December last year, it allows people to monitor the processing of their permit applications in real-time. To achieve successful crowdsourcing projects, the city developed a well-structured work flow to provide explanation of the their goal, accurately define the issues, set up a clear development guideline, clarify the submission requirement and their intended outcome. 
The crowdsourcing practice of the City of Boston shows the potential of crowdsourcing and the strategies to achieve the goal successfully. Eventually crowdsourcing isn’t necessarily to build whatever the public wants, but is to make good use of the public diversity and collective intelligence to enhance city operation and practice.
 Sam Sturgis, Why Crowdsourcing City Projects Actually Works for Boston, Atlantic CITYLAB, May 13, 2015 Retrieved from http://www.citylab.com/tech/2015/03/why-crowdsourcing-city-projects-actually-works-for-boston/387673/
 Steve Annear, Boston looking for ideas on snow disposal, Boston Globe, February 10, 2015. Retrieved from http://www.bostonglobe.com/metro/2015/02/10/what-will-with-all-snow-boston-looking-for-suggestions/o5fAM4PHPlXnVC3OH6PmHJ/story.html
 Mohana Ravindranath, City of Boston signs up for Reston app developer’s Permit Finder, The Washington Post, December 14, 2014 Retrieved from http://www.washingtonpost.com/business/on-it/city-of-boston-signs-up-for-reston-app-developers-permit-finder/2014/12/13/929292c4-8171-11e4-9f38-95a187e4c1f7_story.html
 Sam Sturgis, Should Cities Give Hackathons Another Look to Improve Digital Infrastructure? Atlantic CITYLAB, December 18, 2014 Retrieved from http://www.citylab.com/tech/2014/12/should-cities-give-hackathons-another-look-to-improve-digital-infrastructure/383848/
The association of neighborhood built environment and children’s physical activity has received growing attention. GIS data have helped assess environmental attributes within arbitrarily delineated neighborhoods, but the geographic boundaries of neighborhoods likely vary across children. A one-half-mile radius around children’s residences is widely used as the assumed neighborhood.
There are, however, no data to support this assumption. This study uses diaries and accelerometry to supplement GPS data to help delineate children’s play neighborhoods around their residences. The study found that children focus on one section of the assumed circled neighborhood and suggested using a quarter-mile radius for children’s play neighborhood.
The detailed space-time-activity trajectories at the individual level based on GPS, travel diary, GIS, and accelerometer are useful for planners to examine and define the playing spaces around children’s homes. We compared actual play neighborhoods of our subjects where most moderate and vigorous activities occurred with the predominant half-mile area using a mix of visual and statistical methods. The results of the study will help planners and physical activity investigators understand the extent of children’s play neighborhoods. This study found that children do not use their neighborhood equally in all directions. Their activities usually focus on only one or two directions or sections within a circled neighborhood, yet often it is built environment attributes within the entire circled neighborhood that are used to determine the association of the built environment with the physical activity or adiposity of youth and adults. Using circled neighborhoods, such as the half-mile radius, and calculating built environment variables based on these circled neighborhoods as done in many previous studies may have misrepresented the neighborhood environment characteristics and their contribution to physical activity by including neighborhood attributes that children do not use or experience. The half-mile radius for people’s neighborhoods used in many studies may be a reasonable assumption for general population, but it tends to be larger than the areas used most often by children. This study recommends using a quarter-mile or three-tenth-mile radius for children’s walking and play neighborhood.
In designing the built environment for children’s physical activities, more studies need to be done to examine the elements of the built environment within a smaller radius than one-half mile, and ideally within a non-circled neighborhood that can better represent children’s play neighborhood.
(Research paper published on Urban Studies, UK. Co-authored with Li Yin, Samina Raja, Xiao Li, Leonard Epstein and James Roemmich. Full paper is available via:http://usj.sagepub.com/content/50/14/2922.full)
(Top image: Street playground in London,UK.Photos Courtesy of Yuan Lai.)