Many have come to believe this, but is it true? I don't perceive all of reality as a strict hierarchy of things constructed out of templates, with "is a" and "has a" relationships between them.
The strict hierarchy thing is more a feature of the class-oriented programming of C++ and its close relatives (an artifact of what made sense when approximating OOP in something merged into the existing syntax and type system of C) than it is inherent to OOP.
Unfortunately, the popularity of C++ and Java means that its hard to tell when people are talking about "Object Oriented" if they mean it in the general sense that existed prior to those languages which they are one of many approaches to facilitating, or if it means it in the narrow, static-class-oriented sense which evolved out of the particular approaches of those languages and their close relatives, which is something different.
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The strict hierarchy thing is more a feature of the class-oriented programming of C++ and its close relatives (an artifact of what made sense when approximating OOP in something merged into the existing syntax and type system of C) than it is inherent to OOP.
Unfortunately, the popularity of C++ and Java means that its hard to tell when people are talking about "Object Oriented" if they mean it in the general sense that existed prior to those languages which they are one of many approaches to facilitating, or if it means it in the narrow, static-class-oriented sense which evolved out of the particular approaches of those languages and their close relatives, which is something different.