The idea of design patterns, which Christopher Alexander conveys in his book, is based on identifying the right forces in the field and adapting to these forces. Another idea from the book is of design resilience, or ability to endure and adapt to future changes. In my personal quest for metrics of good design, applicability of resilience has always been under question. How do I know what this block has to be resilient to? Who can predict future changes the program may undergo? That question is dealt with in the book. To know the right answer, one needs to know the forces that define the system, the forces the existing design addresses.