An interesting historical snippet from p48 of psychiatrist Giovanni Stanghellini’s book on the phenomenology of psychosis, Disembodied Spirits and Deanimated Bodies:
The German psychiatrist Karl Willmanns, who would later be director of the Heidelberg Clinic until the rise of the Nazis, published a book [in 1906] on the disenfranchised. He had been following them around at night in the outskirts of town, dressed as one of them, often inviting them into his own home, and ‘lending’ them money.
In his book, Zur Psychopathologie de Landstreichers, Willmanns sought to show how many of the homeless were schizophrenic. His university post, then the most important in German psychiatry, was taken from him, apparently because he diagnosed a form of hysterical blindness in Adolf Hitler.
The book itself is concerned with exploring psychosis using the philosophical tradition of phenomenology, which attempts to carefully describe and understand the structure of subjective consciousness.
Needless to say, this is particularly important so scientific studies can aim to understand what it is important to try and measure in conscious experience, not just attempt to study what is easily measurable.
However, not everyone believes that our own subjective experience is necessarilly a reliable guide to even the conscious mind.
Philosopher Eric Schwitzgebel is a particularly strong critic, suggesting that ‘naive introspection’ is inherently flawed. His debate with psychologist Russell Hurlburt, who disagrees, was recently published as a book.