Here's why:
* It represents a concept: Hierarchy refers to a system of ranking or ordering, not a concrete object.
* It's intangible: You can't physically touch or see a hierarchy. It's an idea or structure.
* It's general and broad: The term "hierarchy" can be applied to various situations, like social structures, organizations, or even biological classifications.
While we can observe hierarchies in the real world (like a company's organizational chart), the concept of hierarchy itself is abstract and exists in our minds.