Speaking more positively, the following remarks may be useful to those who have a mind to observe children:
1. In the first place, we can fix no absolute time in the history of the child at which a certain mental process takes its rise. The observations, now quite extensively recorded, and sometimes quoted as showing that the first year, or the second year, etc., brings such and such developments, tend, on the contrary, to show that such divisions do not hold in any strict sense. Like any other organic growth, the nervous system may develop faster under more favourable conditions, or more slowly under less favourable; and the growth of the mind is largely dependent upon the growth of the brain. Only in broad outline and within very wide limits can such periods be marked off at all.
2. The possibility of the occurrence of a mental state at a particular time must be distinguished from its necessity. The occurrence of a single clearly observed fact is decisive only against the theory according to which its occurrence under the given conditions may not occur. For example, the very early adaptive movements of the infant in receiving its food can not be due to intelligence and will; but the case is still open as to the question what is the reason of their presence--i.e., how much nervous development is present, how much experience is necessary, etc. 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.