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 training of the one-year-old and of the two-year-old child must be, so far as possible, prevented. I have in this respect been so far successful that my child was not until late acquainted with such tricks as children are taught, and was not vexed with the learning by heart of songs, etc., which he was not capable of understanding. Still, as the record shows, not all unnecessary training could be avoided. The earlier a little child is constrained to perform ceremonious and other conventional actions, the meaning of which is unknown to him, so much the earlier does he lose the poetic naturalness which, at any rate, is but brief and never comes again; and so much the more difficult becomes the observation of his unadulterated mental development.
5. Every interruption of one's observation for more than a day demands the substitution of another observer, and, after taking up the work again, a verification of what has been perceived and noted down in the interval.
6. Three times, at least, every day the same child is to be observed, and everything incidentally noticed is to be put upon paper, no less than that which is methodically ascertained with reference to definite questions.
In accordance with these directions, tested by myself, all my own observations in this book, and particularly those of this chapter, were conducted. Comparison with the statements of others can alone give them a general importance.
What has been furnished by earlier observers in regard to children's learning to speak is, however, not extensive. I have collected some data in an appendix.