Introduction.- Three Simple Models.- What can be Learnt from Simple Models?.- Reality Check for Simple Models.- A Physical Simple Model.- What are Simple Models Worth for?.- Complex Models to Understand Complex Social Situations.- Modelling Epidemics.- Why Weather/Climate Forecasts can be Trusted.- We are not Social Atoms.- Social Data are Soaked by Social Complexity.- Machines that Learn How to Model.- Starting from Data to Hunt Causes.- A Moral Thermometer?.- Where do Indicators Lead us?. Which Numbers for the Future?.- Conclusion.- Acknowledgements.- Annexes.