If you're serious about Jungian psychology, I recommend reading the full of Jung's Psychological types because he explains a lot of the premises and concepts that went into his psychological types, but from Chapter 10, which is all I can find online, that's exactly how he defines them -
Classics in the History of Psychology -- Jung (1921/1923) Chapter 10
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Extroverted Type
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Everyone is, admittedly, orientated by the data with which the outer world provides him; yet we see that this may be the case in a way that is only relatively decisive. Because it is cold out of doors, one man is persuaded to wear his overcoat, another from a desire to become hardened finds this unnecessary; one man admires the new tenor because all the world admires him, another withholds his approbation not because he dislikes him but because in his view the subject of general admiration is not thereby proved to be admirable; one submits to [p. 417] a given state of affairs because his experience argues nothing else to be possible, another is convinced that, although it has repeated itself a thousand times in the same way, the thousand and first will be different. The former is orientated by the objective data; the latter reserves a view, which is, as it were, interposed between himself and the objective fact. Now, when the orientation to the object and to objective facts is so predominant that the most frequent and essential decisions and actions are determined, not by subjective values but by objective relations, one speaks of an extraverted attitude. When this is habitual, one speaks of an extraverted type. If a man so thinks, feels, and acts, in a word so lives, as to correspond directly with objective conditions and their claims, whether in a good sense or ill, he is extraverted. His life makes it perfectly clear that it is the objective rather than the subjective value which plays the greater role as the determining factor of his consciousness. He naturally has subjective values, but their determining power has less importance than the external objective conditions. Never, therefore, does he expect to find any absolute factors in his own inner life, since the only ones he knows are outside himself. Epimetheus-like, his inner life succumbs to the external necessity, not of course without a struggle; which, however, always ends in favour of the objective determinant. His entire consciousness looks outwards to the world, because the important and decisive determination always comes to him from without. But it comes to him from without, only because that is where he expects it. All the distinguishing characteristics of his psychology, in so far as they do not arise from the priority of one definite psychological function or from individual peculiarities, have their origin in this basic attitude. Interest and attention follow objective happenings and, primarily, those of the immediate environment. Not [p. 418] only persons, but things, seize and rivet his interest. His actions, therefore, are also governed by the influence of persons and things. They are directly related to objective data and determinations, and are, as it were, exhaustively explainable on these grounds. Extraverted action is recognizably related to objective conditions. In so far it is not purely reactive to environmental stimuli, it character is constantly applicable to the actual circumstances, and it finds adequate and appropriate play within the limits of the objective situation. It has no serious tendency to transcend these bounds. The same holdsgood for interest: objective occurrences have a well-nigh inexhaustible charm, so that in the normal course the extravert's interest makes no other claims.
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C. THE INTROVERTED TYPE
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As I have already explained in section A (1) of the present chapter, the introverted is distinguished from the extraverted type by the fact that, unlike the latter, who is prevailingly orientated by the object and objective data, he is governed by subjective factors. In the section alluded to I mentioned, inter alia, that the introvert interposes a subjective view between the perception of the object and his own action, which prevents the action from assuming a character that corresponds with the objective situation. Naturally, this is a special case, mentioned by way of [p. 472] example, and merely intended to serve as a simple illustration. But now we must go in quest of more general formulations.
Introverted consciousness doubtless views the external conditions, but it selects the subjective determinants as the decisive ones. The type is guided, therefore, by that factor of perception and cognition which represents the receiving subjective disposition to the sense stimulus. Two persons, for example, see the same object, but they never see it in such a way as to receive two identically similar images of it. Quite apart from the differences in the personal equation and mere organic acuteness, there often exists a radical difference, both in kind and degree, in the psychic assimilation of the perceived image. Whereas the extraverted type refers pre-eminently to that which reaches him from the object, the introvert principally relies upon that which the outer impression constellates [sic] in the subject. In an individual case of apperception, the difference may, of course, be very delicate, but in the total psychological economy it is extremely noticeable, especially in the form of a reservation of the ego. Although it is anticipating somewhat, I consider that point of view which inclines, with Weininger, to describe this attitude as philautic, or with other writers, as autoerotic, egocentric, subjective, or egoistic, to be both misleading in principle and definitely depreciatory. It corresponds with the normal bias of the extraverted attitude against the nature of the introvert. We must not forget-although extraverted opinion is only too prone to do so-that all perception and cognition is not purely objective: it is also subjectively conditioned. The world exists not merely in itself, but also as it appears to me. Indeed, at bottom, we have absolutely no criterion that could help us to form a judgment of a world whose nature was unassimilable by the subject. If we were to ignore the subjective factor, it [p. 473] would mean a complete denial of the great doubt as to the possibility of absolute cognition. And this would mean a rechute into that stale and hollow positivism which disfigured the beginning of our epoch -- an attitude of intellectual arrogance that is invariably accompanied by a crudeness of feeling, and an essential violation of life, as stupid as it is presumptuous. Through an overvaluation of the objective powers of cognition, we repress the importance of the subjective factor, which simply means the denial of the subject. But what is the subject? The subject is man -- we are the subject. Only a sick mind could forget that cognition must have a subject, for there exists no knowledge and, therefore, for us, no world where 'I know' has not been said, although with this statement one has already expressed the subjective limitation of all knowledge.
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