Jung did not distinguish between the ISTJ and ISFJ types. Instead he focused on what was common to these two types as both were dominated by Introverted Sensing (Si). Jung has been noted by many to be a forbidding and inaccessible writer and reading him is indeed difficult.
Jung’s Portrait of the ISTJ / ISFJ Types
by Carl Gustav Jung, first published 1921
Introverted Sensation (Si) in general
Sensation, which in obedience to its whole nature is concerned with the object and the objective stimulus, also undergoes a considerable modification in the introverted attitude. It, too, has a subjective factor, for beside the object sensed there stands a sensing subject, who contributes his subjective disposition to the objective stimulus. In the introverted attitude sensation is definitely based upon the subjective portion of perception. What is meant by this finds its best illustration in the reproduction of objects in art. When, for instance, several painters undertake to paint one and the same landscape, with a sincere attempt to reproduce it faithfully, each painting will none the less differ from the rest, not merely by virtue of a more or less developed ability, but chiefly because of a different vision; there will even appear in some of the paintings a decided psychic variation, both in general mood and in treatment of colour and form. Such qualities betray a more or less influential co-operation of the subjective factor. The subjective factor of sensation is essentially the same as in the other functions already spoken of. It is an unconscious disposition, which alters the sense-perception at its very source, thus depriving it of the character of a purely objective influence. In this case, sensation is related primarily to the subject, and only secondarily to the object. How extraordinarily strong the subjective factor can be is shown most clearly in art. The ascendancy of the subjective factor occasionally achieves a complete suppression of the mere influence of the object; but none the less sensation remains sensation, although it has come to be a perception of the subjective factor, and the effect of the object has sunk to the level of a mere stimulant. Introverted sensation develops in accordance with this subjective direction. A true sense-perception certainly exists, but it always looks as though objects were not so much forcing their way into the subject in their own right as that the subject were seeing things quite differently, or saw quite other things than the rest of mankind. As a matter of fact, the subject perceives the same things as everybody else, only, he never stops at the purely objective effect, but concerns himself with the subjective perception released by the objective stimulus. Subjective perception differs remarkably from the objective. It is either not found at all in the object, or, at most, merely suggested by it; it can, however, be similar to the sensation of other men, although not immediately derived from the objective behaviour of things. It does not impress one as a mere product of consciousness — it is too genuine for that. But it makes a definite psychic impression, since elements of a higher psychic order are perceptible to it. This order, however, does not coincide with the contents of consciousness. It is concerned with presuppositions, or dispositions of the collective unconscious, with mythological images, with primal possibilities of ideas. The character of significance and meaning clings to subjective perception. It says more than the mere image of the object, though naturally only to him for whom the subjective factor has some meaning. To another, a reproduced subjective impression seems to suffer from the defect of possessing insufficient similarity with the object; it seems, therefore, to have failed in its purpose. Subjective sensation apprehends the background of the physical world rather than its surface. The decisive thing is not the reality of the object, but the reality of the subjective factor, i.e. the primordial images, which in their totality represent a psychic mirror-world. It is a mirror, however, with the peculiar capacity of representing the present contents of consciousness not in their known and customary form but in a certain sense sub specie aeternitatis, somewhat as a million-year old consciousness might see them. Such a consciousness would see the becoming and the passing of things beside their present and momentary existence, and not only that, but at the same time it would also see that Other, which was before their becoming and will be after their passing hence. To this consciousness the present moment is improbable. This is, of course, only a simile, of which, however, I had need to give some sort of illustration of the peculiar nature of introverted sensation. Introverted sensation conveys an image whose effect is not so much to reproduce the object as to throw over it a wrapping whose lustre is derived from age-old subjective experience and the still unborn future event. Thus, mere sense impression develops into the depth of the meaningful, while extraverted sensation seizes only the momentary and manifest existence of things.
The Introverted Sensation [ISTJ / ISFJ] Types
The priority of introverted sensation produces a definite type, which is characterized by certain peculiarities. It is an irrational type, inasmuch as its selection among occurrences is not primarily rational, but is guided rather by what just happens. Whereas, the extraverted sensation-type is determined by the intensity of the objective influence, the introverted type is orientated by the intensity of the subjective sensation-constituent released by the objective stimulus. Obviously, therefore, no sort of proportional relation exists between object and sensation, but something that is apparently quite irregular and arbitrary judging from without, therefore, it is practically impossible to foretell what will make an impression and what will not. If there were present a capacity and readiness for expression in any way commensurate with the strength of sensation, the irrationality of this type would be extremely evident. This is the case, for instance, when the individual is a creative artist. But, since this is the exception, it usually happens that the characteristic introverted difficulty of expression also conceals his irrationality. On the contrary, he may actually stand out by the very calmness and passivity of his demeanour, or by his rational self-control. This peculiarity, which often leads the superficial judgment astray, is really due to his unrelatedness to objects. Normally the object is not consciously depreciated in the least, but its stimulus is removed from it, because it is immediately replaced by a subjective reaction, which is no longer related to the reality of the object. This, of course, has the same effect as a depreciation of the object. Such a type can easily make one question why one should exist at all; or why objects in general should have any right to existence, since everything essential happens without the object. This doubt may be justified in extreme cases, though not in the normal, since the objective stimulus is indispensable to his sensation, only it produces something different from what was to be surmised from the external state of affairs. Considered from without, it looks as though the effect of the object did not obtrude itself upon the subject. This impression is so far correct inasmuch as a subjective content does, in fact, intervene from the unconscious, thus snatching away the effect of the object. This intervention may be so abrupt that the individual appears to shield himself directly from any possible influence of the object. In any aggravated or well-marked case, such a protective guard is also actually present. Even with only a slight reinforcement of the unconscious, the subjective constituent of sensation becomes so alive that it almost completely obscures the objective influence. The results of this are, on the one hand, a feeling of complete depreciation on the part of the object, and, on the other, an illusory conception of reality on the part of the subject, which in morbid cases may even reach the point of a complete inability to discriminate between the real object and the subjective perception. Although so vital a distinction vanishes completely only in a practically psychotic state, yet long before that point is reached subjective perception may influence thought, feeling, and action to an extreme degree, in spite of the fact that the object is clearly seen in its fullest reality. Whenever the objective influence does succeed in forcing its way into the subject — as the result of particular circumstances of special intensity, or because of a more perfect analogy with the unconscious image — even the normal example of this type is induced to act in accordance with his unconscious model. Such action has an illusory quality in relation to objective reality, and therefore has a very odd and strange character. It instantly reveals the anti-real subjectivity of the type, But, where the influence of the object does not entirely succeed, it encounters a benevolent neutrality, disclosing little sympathy, yet constantly striving to reassure and adjust. The too-low is raised a little, the too-high is made a little lower; the enthusiastic is damped, the extravagant restrained; and the unusual brought within the ‘correct’ formula: all this in order to keep the influence of the object within the necessary bounds. Thus, this type becomes an affliction to his circle, just in so far as his entire harmlessness is no longer above suspicion. But, if the latter should be the case, the individual readily becomes a victim to the aggressiveness and ambitions of others. Such men allow themselves to be abused, for which they usually take vengeance at the most unsuitable occasions with redoubled stubbornness and resistance. When there exists no capacity for artistic expression, all impressions sink into the inner depths, whence they hold consciousness under a spell, removing any possibility it might have had of mastering the fascinating impression by means of conscious expression. Relatively speaking, this type has only archaic possibilities of expression for the disposal of his impressions; thought and feeling are relatively unconscious, and, in so far as they have a certain consciousness, they only serve in the necessary, banal, every-day expressions. Hence as conscious functions, they are wholly unfitted to give any adequate rendering of the subjective perceptions. This type, therefore, is uncommonly inaccessible to an objective understanding and he fares no better in the understanding of himself.
Above all, his development estranges him from the reality of the object, handing him over to his subjective perceptions, which orientate his consciousness in accordance with an archaic reality, although his deficiency in comparative judgment keeps him wholly unaware of this fact. Actually he moves in a mythological world, where men animals, railways, houses, rivers, and mountains appear partly as benevolent deities and partly as malevolent demons. That thus they, appear to him never enters his mind, although their effect upon his judgments and acts can bear no other interpretation. He judges and acts as though he had such powers to deal with; but this begins to strike him only when he discovers that his sensations are totally different from reality. If his tendency is to reason objectively, he will sense this difference as morbid; but if, on the other hand, he remains faithful to his irrationality, and is prepared to grant his sensation reality value, the objective world will appear a mere make-belief and a comedy. Only in extreme cases, however, is this dilemma reached. As a rule, the individual acquiesces in his isolation and in the banality of the reality, which, however, he unconsciously treats archaically.
His unconscious is distinguished chiefly by the repression of intuition, which thereby acquires an extraverted and archaic character. Whereas true extraverted intuition has a characteristic resourcefulness, and a ‘good nose’ for every possibility in objective reality, this archaic, extraverted intuition has an amazing flair for every ambiguous, gloomy, dirty, and dangerous possibility in the background of reality. In the presence of this intuition the real and conscious intention of the object has no significance; it will peer behind every possible archaic antecedent of such an intention. It possesses, therefore, something dangerous, something actually undermining, which often stands in most vivid contrast to the gentle benevolence of consciousness. So long as the individual is not too aloof from the object, the unconscious intuition effects a wholesome compensation to the rather fantastic and over credulous attitude of consciousness. But as soon as the unconscious becomes antagonistic to consciousness, such intuitions come to the surface and expand their nefarious influence: they force themselves compellingly upon the individual, releasing compulsive ideas about objects of the most perverse kind. The neurosis arising from this sequence of events is usually a compulsion neurosis, in which the hysterical characters recede and are obscured by symptoms of exhaustion.