Human brains love either/or categories.
Good/evil, left/right, man/woman, this moiety 
vs. that one (the only other one), child/adult, handsome/ugly, liked/disliked, loyal/traitorous, theory/experiment, academic/practical, etc. etc. etc.
As the examples show, we often refer to an element that we *know* lies somewhere on a continuum as if it can be described well enough with one bit of information: is she good or evil?
This doesn't appear just to be mental shorthand; people will strongly resist having to contemplate a continuum. And they frequently reject the notion that the two extremes might have a complicated relationship.
Binary oppositions are frequently organized in a Binary Hierarchy.