Jaspers set down his views on mental illness in a book which he published in 1913, "General Psychopathology". This work has become a classic in the psychiatric literature and many modern diagnostic criteria stem from ideas found within it. One of Jaspers' central tenets was that psychiatrists should diagnose symptoms of mental illness (particularly of psychosis) by their "form" rather than by their "content". For example, in diagnosing a hallucination, it is more important to note that a person experiences visual phenomena when no sensory stimuli account for them, than to note what the patient sees. What the patient sees is the "content", but the discrepancy between visual perception and objective reality is the "form".