If you’re a programmer you make design decisions every second. Statements, functions, classes, packages, applications, even entire systems: you need to think, and often think hard, about everything. Luckily there are many useful design principles, patterns and best practices that you can apply. But some of them merely expose code smells. Others only help you design your classes. And some are applicable to packages only. Wouldn’t it be nice to have some more general, always useful, invariably applicable, foundational design principles? In this talk we’ll look at software from many different perspectives, and while we’re zooming in and out, we’ll discover some of the deeper principles that lie beneath proper object-oriented design. They are the foundation of many of the well-known design patterns and they may even serve as an explanation for code smells.