In saying this, it is obvious that contact with both the external world and the self are necessary factors in all living creatures. Even with animals this is evident. If they were to react to external stimuli only, they would become completely exhausted under certain conditions; while under other circumstances they would show no activity at all. If, on the other hand, they were to respond to inner needs without any regard to circumstances, they would probably soon perish. In man both influences are to be found in the conscious personality, but the distinctness with which their effects are seen varies considerably, according to whether the individual is predominantly introverted or extraverted.
Once this difference is recognized, there is usually little difficulty in finding extreme cases of the two types. There are outstandingly extraverted persons, who are almost exclusively led by impressions and impulses aroused in them from the outside. They are, as a result, lively and changeable, and they really do not know themselves at all. On the other hand, extremely introverted people entrench themselves against the external world, and after the slightest contact with it quickly withdraw into themselves again. They impress one as withdrawn and shy.
Even where the general attitude is less marked, it is usually possible, when things are difficult, to note which form of adaptation is the predominating one. The extravert, when uncertain, conforms to the opinion of other people, and immediately becomes more lively and freer in his activity. Under similar circumstances, the introvert is more inclined to withdraw into himself, seeking a solution there. The way in which they take life may also occasionally lead us to a correct diagnosis as to type. The extravert seeks, above all things, to remain in harmony with his environment; the introvert seeks harmony in himself.