In The Content of the Psychoses (1908), Jung presents case studies as proof that the purely anatomical approach to psychoses leads only indirectly to an understanding of psychic disturbances. He believes that many psychoses, and particularly schizophrenia, were psychogenic, resulting from an abaissement du niveau mental - a reduced state of concentration and attention, accompanied by a loosening of inhibitions and relaxation of restraints, in which unexpected contents may emerge from the collective unconscious. In such a case, the ego is too weak to resist the onslaught of unconscious contents.
Jung reserved judgment on whether biological factors were a contributing cause. He laments the fact that psychiatry is the "stepchild of medicine" because the scientific methods useful in other branches of medicine cannot be applied to the mind.