  Several approaches and corresponding definitions of complexity have been
developed in different fields. Urban systems are the archetype of complex
socio-technical systems concerned with these different viewpoints. We suggest
in this chapter some links between three types of complexity, namely emergence,
computational complexity and informational complexity. We discuss the
implication of these links on the necessity of reflexivity to produce a
knowledge of the complex, and how this connects to the interdisciplinary of
approaches in particular for socio-technical systems. We finally synthesize
this positioning as a proposal of an epistemological framework called applied
perspectivism, and discuss the implications for the study of urban systems.
