Information is related to each other. Relationships appear to be hierarchical because of the lens the information is being looked at.

Hearts pertain to biology, are part of any animal, and are an oft-used metaphor for emotions. They are made up of molecules, and that's part of chemistry. They are ruled by quantum mechanics, and that's part of physics.

One can certainly arbitrarily place the concept of a "heart" into one of these buckets and say "that's where it belongs", but I think that perspective is arbitrary.

I posit that a more useful perspective is to look at the various connections between a given topic or concept and see what they are linked to. The heart example is thus updated: all of the listed topics are within the category of "heart."