It is well to emphasize the fact that one case may be decisive in overthrowing a theory, but the conditions are seldom simple enough to make one case decisive in establishing a theory.
3.

It follows, however, from the principle of growth itself that the order of development of the main mental functions is constant, and normally free from great variations; consequently, the most fruitful observations of children are those which show that such an act was present _before another_.

The complexity becomes finally so remarkable that there seems to be no before or after at all in mental things; but if the child's growth shows a stage in which any process is clearly absent, we have at once light upon the laws of growth.

For instance: if a single case is conclusively established of a child's drawing an inference before it begins to use words or significant vocal sounds, the one case is as good as a thousand to show that thought may develop in some degree independently of spoken language.
4.

While the most direct results are acquired by systematic experiments with a given point in view, still general observations carefully recorded by competent persons, are important for the interpretation which a great many such records may afford in the end.
In the multitude of experiences here, as everywhere, there is strength.