"On studying closely the etiology of mental diseases one soon recognizes the fact that in the great majority of cases the disease is produced - not by a particular or specific cause, but by a series of unfavorable conditions which first prepare the soil and then, by their simultaneous action, determine the outbreak of insanity." 1
This was written nearly three-quarters of a century ago. To-day, though this view is still held to a certain extent, we are nevertheless able to distinguish amongst the many causes some few that are essential from others that are merely incidental or contributing. In addition there are other factors that have to do with the etiology of mental disorders, especially, race, age, sex, environment, occupation, marital condition, education, and immigration.
As implied in the term itself, the essential causes are those in the absence of which mental disorders do not occur.
1 Gricsinger. Die Pathologie und Therapie der Geisteskrankheiten.
Each of these alone may suffice to produce a mental disorder or it may act by rendering the nervous organization so vulnerable that a breakdown occurs at the occasion of some incidental cause which may be in itself quite insignificant but which here comes to play the part of "the last straw that broke the camel's back."