@Orannis62 already posted a really informative comment about it. I would just like to add that a huge part of the focus is on the symptoms particularly because we don't have a good understanding as to what schizophrenia really is. There are different ways in which schizophrenia presents itself. In our lab meeting today, we wondered if the category "schizophrenia" is even useful anymore.

For example, the philosopher/phenomenologist Shaun Gallagher has asked several important questions like, "Why do schizophrenics experience some inner speech as an auditory hallucination and some not?" Now, this rests on the assumption that some auditory hallucinations (like thought echo) are actually a confusion about where inner speech comes from (incorrectly perceiving/thinking it as coming from an external source). Another important question is, "Why do some schizophrenia patients experience delusions of control and another type of First Rank Symptom?"