Medical experience has taught us that there are two large groups of functional nervous disorders. The one embraces all those forms of disease which are designated _hysterical_, the other all those forms which the French school has designated _psychasthenic_. Although the line of demarcation is rather uncertain, one can mark off two psychological types which are obviously different; their psychology is diametrically opposed. I have called these--the _Introverted_ and _Extroverted_ types. The hysteric belongs to the type of _Extroversion_, the psychasthenic to the type of _Introversion_, as does dementia præcox, in so far as we know it to-day. This terminology, _Introversion_ and _Extroversion_, is bound up with my way of regarding mental phenomena as forms of energy. I postulate a hypothetical fundamental striving which I designate _libido_.[213] In the classical use of the word, _libido_ never had an exclusively sexual connotation as it has in medicine. The word _interest_, as Claparède once suggested to me, could be used in this special sense, if this expression had to-day a less extensive application. Bergson's concept, _élan vital_, would also serve if this expression were less biological and more psychological. Libido is intended to be an energising expression for _psychological values_. The psychological value is something active and determining; hence it can be regarded from the energic standpoint without any pretence of exact measurement. The introverted type is characterised by the fact that his libido is turned towards his own personality to a certain extent--he finds within himself the unconditioned value. The extroverted type has his libido to a certain extent externally; he finds the unconditioned value outside himself. The introvert regards everything from the aspect of his own personality; the extrovert is dependent upon the value of his object. I must emphasise the statement that this question of types is _the_ question of our psychology, and that every further advance must probably proceed by way of this question. The difference between these types is almost alarming in extent. So far there is only one small preliminary communication by myself[214] on this theory of type, which is particularly important for the conception of dementia præcox. On the psychiatric side Gross[215] has called attention to the existence of two psychological types. His two types are (1) those with limited but deep consciousness, and (2) those with broad but superficial consciousness. The former correspond to my introverted and the latter to my extroverted type. In my article I have collected some other instances among which I would especially call attention to the striking description of the two types given by William James in his book on "Pragmatism." Fr. Th. Vischer has differentiated the two types very wittily by her division of the learned into "reason-mongers," and "matter-mongers." In the sphere