The word empirical (according to the Cambridge English dictionary) means based on what is experienced or seen, rather than on theory. This is perhaps the critical principle to be followed. A fascinating article by Gustavo Duarte (mentioned here: https://www.infoq.com/news/2008/02/realitydrivendevelopment) states: "Action and experimentation are the cornerstones of empiricism. No attempt is made to subdue reality by extensive analysis and copious documentation. Reality is invited in via experiments. Instead of agonizing over market research, an empirical company hires interns and develops a product in one summer. A non-empirical company has 43 people planning an off-button design for one year." Throughout this book, too, we have always tried to consciously follow an empirical approach; we definitely urge the reader to cultivate and embed the empirical principle in design and development.