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.