he was three years old approximated
more and more rapidly to that of the family through continued listening
to them and imitation of them, so that I gave up recording it; besides,
the abundant--some may think too abundant--material already presented
supplies facts enough to support the foundations of the history of the
development of speech in the child as I have attempted to set it forth.
A systematic, thorough-going investigation requires the combined labor
of many, who must all strive to answer the same questions--questions
which in this chronological survey are, in regard to one single
individual, in part answered, but in part could merely be proposed.

To observe the child every day during the first thousand days of his
life, in order to trace the historical development of speech, was
possible only through self-control, much patience, and great expenditure
of time; but such observations are necessary, from the physiological,
the psychological, the linguistic, and the pedagogic point of view, and
nothing can supply their place.

In order to secure for them the highest degree of trustworthiness, I
have adhered strictly, without exception, to the following rules:

1. I have not adopted a single observation of the accuracy of which I
was not _myself_ most positively convinced. Least of all can one rely on
the reports of nurses, attendants, and other persons not practiced in
scientific observing. I have often, merely by a brief, quiet
cross-examination, brought such persons to see for themselves the
erroneous character of their statements, particularly in case these were
made in order to prove how "knowing" the infants were. On the other
hand, I owe to the mother of my child, who has by nature a talent for
observation such as is given to few, a great many communications
concerning his mental development which have been easily verified by
myself.

2. Every observation must _immediately_ be entered in writing in a diary
that is always lying ready. If this is not done, details of the
observations are often forgotten; a thing easily conceivable, because
these details in themselves are in many ways uninteresting--especially
the meaningless articulations--and they acquire value only in connection
with others.

3. In conducting the observations every artificial strain upon the child
is to be avoided, and the effort is to be made as often as possible to
observe without the child's noticing the observer at all.

4. All t