This. I read an article some months ago about sub-types of "schizophrenia" and the exponential growth the number of identified and possibly completely new ones psychology has seen since the 70s. And that is only one among hundreds of broad categories for mental illnesses.
Even for Bipolar Affective Disorder*, which has only two or three subtypes, there are plenty of variations between individual patients. Not just differences between the subtypes, but between each person. Not everyone has every possible symptom, not every person with a certain symptom has it to the same severity. Its still clearly the same illness, but each case is at least slightly different. Bit like humans in general, for some reason :whistling:.
*I like using the longer name, because B.A.D. makes a way cooler acronym than 'BP'

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