[proverbialbunny]

It's unfortunately a complex issue.

There are multiple kinds of depression. Some qualitative, some quantitative, and some a mix.

We categorize diseases by their symptoms, but multiple different diseases can have identical symptoms. When this happens, we often label that group of symptoms as one disease. We start to give it multiple different names once we have multiple kinds of treatments, each treating each kind. Psychology has this problem where multiple diseases have overlapping symptoms, and because we haven't found out a single magic bullet, we often think of depression as one thing today, when really it should be thought of as a myriad of different diseases that each give a similar profile of symptoms.