The second book is about modularity. Well designed or evolved structures are often not just made of parts, but made of parts with simple interfaces. These interfaces allow the parts to be reused in alternative contexts, and thus recombined in different ways.

One of the most important benefits is inspection and debugging. Modular systems are easier to understand, because their components are self-contained. However, this can also be a curse; when components have boundaries that are simple relative to their internals, they cannot share all of their information with the other components, which can lead to communication and coordination failures.

The essays in this collection have been split into four clusters, to make them easier to and and remember. This book contains essays related to modularity, both as a thing that we engineer, and as a thing that we discover in the world.

This book will explore modularized systems through topics such as biology, machine learning, companies, human emotion, and more.