Following is Jung’s description from the Classics in the History of Psychology. For those who have found hid work hard to followd, I have edited the information without hopefully diluting the meaning. Below is a syllabus of words that have been changed per my own translation. If you have read Jung and see that my translation is in error, please let me know:
Conscious = Dominant Function
Unconscious = Auxiliary Function
Attitude = Extravert/Introvert
Object = External
Subject = Internal
GENERAL DESCRIPTION OF THE TYPES
In the following pages I shall attempt a general description of the types, and my first concern must be with the two general types I have termed introverted and extraverted. But in addition, I shall also try to give a certain characterization of those special types whose particularity is due to the fact that his most differentiated function plays the principal role in an individual's adaptation or orientation to life. The former I would term general attitude types, since they are distinguished by the direction of general interest or energy movement, while the latter I would call function-types.
The general-attitude types, as I have pointed out more than once, are differentiated by their particular attitude to the external world. The introvert's attitude to the external is an abstracting one. At the very least he is always facing the problem of how their energy can be withdrawn from the external world, as though an attempted ascendancy on the part of the external had to be continually frustrated. The extravert on the contrary, maintains a positive relation to the external world. To such an extent does he affirm its importance that his extraverted attitude is continually being orientated by and related to the external. The internal can never have sufficient value for him therefore its importance must always be paramount.
The two types are so essentially different, presenting so striking a contrast that their existence even to the uninitiated in psychological matters becomes an obvious fact, when once attention has been drawn to it. Who does not know those taciturn, impenetrable, often shy natures, who form such a vivid contrast to these other open, sociable, serene maybe, or at least friendly and accessible characters, who are on good terms with all the world, or, even when disagreeing with it, still hold a relation to it by which they and it are mutually affected.
Naturally at first, one is inclined to regard such differences as mere individual idiosyncrasies. But anyone with the opportunity of gaining a fundamental knowledge of many men will soon discover that such a far-reaching contrast does not merely concern the individual case, but is a question of typical attitudes with a universality far greater than a limited psychological experience would at first assume. In reality it is a question of a fundamental opposition, at times clear and at times obscure but always emerging whenever we are dealing with individuals whose personality is in any way pronounced. Such men are found not only among the educated classes but in every rank of society; with equal distinctness. Therefore our types can be demonstrated among laborers and peasants as among the most differentiated members of a nation.
Furthermore, these types over-ride the distinctions of sex since one finds the same contrasts amongst women of all classes. Such a universal distribution could hardly arise at the instigation of consciousness, as the result of a conscious and deliberate choice of attitude. If this were the case, a definite level of society linked together by a similar education and environment and therefore, correspondingly localized, would surely have a majority representation of such an attitude. But the actual facts are just the reverse, for the types have apparently, quite a random distribution. In the same family one child is introverted, and another extraverted.
Since, in the light of these facts, the attitude-type regarded as a general phenomenon having an apparent random distribution, can be no affair of conscious judgment or intention. Its existence must be due to some unconscious instinctive cause. The contrast of types, as a universal psychological phenomenon, must in some way or other have its biological precursor.
The relation between the internal and the external, considered biologically, is always a relation of adaptation since every relation between subject and object presupposes mutually modifying effects from either side. These modifications constitute the adaptation. The typical attitudes to the external world therefore are adaptation processes. Nature knows two fundamentally different ways of adaptation that determine the further existence of the living organism the one is by increased fertility, accompanied by a relatively small degree of defensive power and individual conservation. The other is by individual equipment of manifold means of self-protection coupled with a relatively insignificant fertility. This biological contrast seems not merely to be the analogue but also the general foundation of our two psychological modes of adaptation. At this point a mere general indication must suffice. On the one hand, I need only point to the peculiarity of the extravert, which constantly urges him to spend and propagate himself in every way; and on the other, to the tendency of the introvert to defend himself against external claims, to conserve himself from any expenditure of energy directly related to the object, thus consolidating for himself the most secure and impregnable position.
The fact that often in their earliest years children display an unmistakable typical attitude forces us to assume that it cannot possibly be the struggle for existence, as it is generally understood which constitutes the compelling factor in favor of a definite attitude. We might however, demur and indeed with cogency, that even the tiny infant, the very babe at the breast, has already an unconscious psychological adaptation to perform, inasmuch as the special character of the maternal influence leads to specific reactions in the child. This argument though appealing to incontestable facts has none-the-less to yield before the equally unarguable fact that two children of the same mother may at a very early age exhibit opposite types without the smallest accompanying change in the attitude of the mother. Although nothing would induce me to underestimate the well-nigh incalculable importance of parental influence, this experience compels me to conclude that the decisive factor must be looked for in the disposition of the child. The fact that in spite of the greatest possible similarity of external conditions, one child will assume this type while another that, must of course in the last resort he ascribed to individual disposition. Naturally in saying this I only refer to those cases that occur under normal conditions. Under abnormal conditions, when there is an extreme and therefore abnormal attitude in the mother, the children can also be coerced into a relatively similar attitude. But this entails a violation of their individual disposition that quite possibly would have assumed another type if no abnormal and disturbing external influence had intervened. As a rule, whenever such a falsification of type takes place as a result of external influence, the individual becomes neurotic later, and a cur can successfully be sought only in a development of that attitude which corresponds with the individual's natural way.
As regards the particular disposition, I know not what to say, except that there are clearly individuals who have either a greater readiness and capacity for one way, or for whom it is more congenial to adapt to that way rather than the other. In the last analysis it may well be that physiological causes, inaccessible to our knowledge, play a part in this. That this may be the case seems to me not improbable, in view of one's experience that a reversal of type often proves exceedingly harmful to the physiological well being of the person, often provoking an acute state of exhaustion.