We opened this book by arguing that mainstream macroeconomics is in troubled methodological and explanatory waters. Throughout it we advocated that a possible way out from this quandary is a bottom-up approach: let us start from the analysis of the behaviour of heterogeneous constitutive elements (defined in terms of simple, observation-based behavioural rules) and their local interactions, and allow for the possibility that interaction nodes and individual rules change in time (adaptation). At the next meso-level, statistical regularities emerge that cannot be inferred from the primitives of individuals (self-emerging regularities). This emergent behaviour feeds back to the individual level (downward causation), but also produces aggregate regularities at the next hierarchical level. High-levels (macroeconomic) systems possess new and different properties than low-level (microeconomic) systems, like water has different properties from the atoms of hydrogen and oxygen that constitute it, as well as from ice and steam, and from the multicellular living organisms containing it. This approach allows each and every proposition to be falsified at micro, meso and macro levels and, de facto, it opposes to the mainstream axiomatic theory of economics, where microeconomic optimization is considered the rule for any rigorous scientific practice.