His instinctive impulses are so greatly masked by the variety of response that his intellect opens to him that he has been commonly regarded until quite recent times as a practically non-instinctive creature, capable of determining by reason his conduct and even his desires. Such a conception made it almost impossible to gain any help in human psychology from the study of other animals, and scarcely less difficult to evolve a psychology which would be of the least use in foreseeing and controlling the behaviour of man.
No understanding of the causes of stability and instability in human society is possible until the undiminished vigour of instinct in man is fully recognized.
The significance of this rich instinctive endowment lies in the fact that mental health depends upon instinct finding a balanced but vigorous expression in functional activity. The response to instinct may be infinitely varied, and may even, under certain circumstances, be not more than symbolic without harm to the individual as a social unit, but there are limits beyond which the restriction of it to indirect and symbolic modes of expression cannot be carried without serious effects on personality. The individual in whom direct instinctive expression is unduly limited acquires a spiritual meagreness which makes him the worst possible social material.