The terminology of mental and nervous disorders has been so loosely
applied that some definition may be necessary. By the term "idiocy,"
is meant a condition of undeveloped mentality. Idiocy exists in
various degrees, from the complete absence of intellectual faculties
to a condition of mere irresponsibility in which the subject is
capable of self-help, and sometimes of self-support under the careful
guidance of other. Under the generic term "idiot" may be included the
"complete idiot," the imbecile, the "feeble-minded" and the
"simpleton," all of whom suffer in a greater or less degree from
arrested mental development.

Insanity, on the other hand, is a disease which destroys or clouds an
intellect which has once been developed. It is true that certain
conditions of idiocy and imbecility do resemble that phase of insanity
known as dementia--a reversion to the original mental state of
childhood--in reality a form of second childhood. But the states are
not identical, although one may lapse into the other. One is defect,
the other disease; the imbecile in the former being the counterpart of
the dement in the latter, just as the moral imbecile is the analogue
of the paranoiac.[61]

[Footnote 61: Barr, _Mental Defectives_, p. 18.]

Of the strong inheritability of idiocy there can be no doubt. Dr.
Martin W. Barr of the Pennsylvania Training School for Feeble Minded
Children has published an etiological table embodying the results of a
careful examination of 4050 cases of mental defect. Of these, 2651 or
65.45 per cent resulted from causes acting before birth, including
1030 or 25.43 per cent with a family history of idiocy and imbecility,
and 529 more (13.06 per cent) with a family history of insanity,