If you’re a programmer you make design decisions every second. You need to think, and often think hard, about everything. Luckily there are many useful design principles and patterns 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 form the basis of proper object-oriented design. After attending this talk you will be able to make better design decisions by reflecting on the stream of messages that is flowing from object to object, and from application to application.