I kind of stopped understanding this explanation of inheritance (when I switched to composition of course ;) ). When I look at the world I see a universe of composition, we get molecules by composing atoms, atoms from composing protons, neutrons, etc. little "modules" that come together. More practically, everyday objects seem to follow this too. A motorcycle and car aren't both "refined" from some "wheeled vehicle" that car companies buy, rather they share some similar components that car companies buy. Their similarities are thus either emergent (interface/protocol) or a direct result of their shared components. In unity for example, you don't inherit every hit testable thing from some class (like you would in cocoa), but rather attach a collider to things that can be hit tested. The collider actually worries about one concern, and you can therefor conveniently attach multiple colliders to one object for example (makes it easy to form complex hit regions). Similarly things that are visible have renderers attached. This matches my intuition of working with real objects, where is buy parts and put them together, not think to myself which "master" part my new thing derives from, purchase it, then rip it open and start modifying it as step 1.