Your domain model is not in your classes, it’s in the communication patterns between the objects at runtime.
Steve Freeman and Nat Pryce, Growing Object-Oriented Software, Guided by Tests, Addison Wesley, 2009.
Procedural code gets information then makes decisions. Object-oriented code tells objects to do things.
Alec Sharp, Smalltalk by Example: The Developer’s Guide, McGraw-Hill, 1997.
Cited by Matt Wynne, Hexagonal Rails, GoRuCo 2012