Roberto Assagioli’s definition of introvert versus extrovert from the article Jung and Psychosynthesis: (See also the article Extroverts and Introverts by Roberto Assagioli)
“We now come to the direction of the vital interest, and so pass from the descriptive to the dynamic aspect. One of Jung’s most valuable contributions was the discovery and description of two fundamental psychological types based on whether the vital interest is directed outwards or inwards, and thus “extraverted” or “introverted”. I should mention at once that it is less a matter of “types” in a precise and static sense, and more of the prevailing direction of the vital interest, and thus of the consequent evaluations, chokes, decisions and actions. This predominating tendency can be strong (for instance, indicating this intensity in percentages—ninety per cent) or weak (sixty per cent, or say forty per cent). There is little need to describe the characteristics of the extravert and introvert; by now they are a matter of common knowledge. It is worth remembering that this prevailing tendency is subject to extreme, even pathological, variations. In its almost pure form, extraversion is to be observed in manic states, introversion in melancholia and depression.
This direction of the vital interest is susceptible to alternations and oscillations ranging from the normal and moderate to the extreme and pathological. The extremes in alternation are to be found in cyclothimia and manic-depressive psychoses, which may or may not be intercalated by periods of equilibrium. In addition, the alternation can be rapid or slow, the cycles long or short. It is interesting to observe how a normal alternation occurs in relation to the various ages from birth to old age. The infant is totally introverted, totally absorbed in his organic sensations. As childhood progresses, he becomes increasingly extraverted and directs his interest towards the external world. The adolescent reverts to introversion when the awakening of energies, feelings, and emotions creates problems and crises that focus his interest upon himself. This generally gives place again to extraversion as the young man and adult become involved in relationships with others (interpersonal and social) and in professional activities. Maturity and especially old age produce a return to introversion, accompanied by detachment and waning interest in the external world, and by a tendency towards the inner life, contemplation and dispassionate observation.
By combining the tendency to extraversion or introversion with the four psychological functions he postulates, Jung arrives at a classification of eight types: the extraverted sensory, the extraverted emotional, the extraverted mental, the extraverted intuitive, and four corresponding introverted types. But this and other classifications expose those who adopt them to the dangers of schematicism and pigeon-holing, of yielding to the (so comfortable!) tendency to “label” human beings. We must be on our guard against overlooking the multifarious and complex facets of human reality. It is all too easy to regard others as “objects” instead of “subjects”. And this labeling, with its associated attitudes of judging, or more often of depreciation, often provokes hostile reactions, sometimes of an intense kind, which are thoroughly justified.
But to the eight types recognized by Jung, others must be added. Opposite interest-directions can be associated simultaneously with different levels in the same personality. For instance, a man may be predominantly extraverted physically, introverted emotionally and again extraverted mentally. His will can also be extraverted or introverted. Furthermore, another distinction must be made: the direction of the vital interest is subject to two separate “modalities” or attitudes: the active and the passive. Jung mentions this, but does not develop the point, which, in my opinion, has a fundamental importance. A passive extravert, endowed with excessive sensitivity, who succumbs to every external influence and is dominated by the will of others, is very different from an active extravert who tends to dominate things and people, to bend them to his will. In this sense, they are opposite types.
To this must be added the fact that there are two other interest-directions to be recognized and given the utmost consideration; the direction downwards towards the low, which may be called subversion, and that upwards towards the high, or supra-version. Subversion is the tendency to plumb the unconscious in its lower aspects; and it can be said to be the province of “depth psychology” in its more restricted sense (the “descent into hell”) and can be compared to sub-aquatic sport. Freudian psychoanalysis displays an almost exclusive interest in the lower aspects of human nature.
In supra-version, on the other hand, the vital interest and search are directed towards the higher aspects of the psyche, towards the superconscious, towards spiritual experiences. This, in contrast to sub-aquatic sport, can be compared to mountain-climbing. To Jung must be given the credit of having recognized and demonstrated the existence in the human being of the natural tendency towards the high, of a genuine need, which he called instinctive, for spiritual satisfaction. He gave prominence to the fact that the neglect or repression of this need can create serious neuro-psychic and psychosomatic disturbances.
Another, and important, difference is one of quality, which is different from direction. There can be a supra-version of an inferior kind: the dreamer, the passive idealist, the sterile theoretician, the utopian are examples of supra-version of a negative type. There is again a subversion of a superior kind, such as the scientific investigation and exploration of the lower aspects of the unconscious, what could be termed psychological geology and archaeology.
Although I cannot now discuss the pschosynthetic tasks connected with the various directions of the vital interest, I should mention that there are also other psychological types deriving from the differences in the personality “structure”. There are individuals who are relatively coherent, well “shaped”, even rigid. 0n the other hand there are others who are diffuse, continually changeable. Others, again, are habitually contradictory or ambivalent.” (C.G. Jung and Psychosynthesis)
“Q. Could you say something of the ambivert? Is there a personality type indicating integration and ambiversion?
A. I’m going to give you the appendix from my book on the Will, on differential psychology. There I made it clear that, in a sense, there are no fixed types. Introvert , extrovert, etc., are not labels. They simply mean a direction of the life interest; that is a process, not something static. Therefore, we should not label either ourselves nor anybody as extrovert or introvert, and we do that all the time and it is a limiting label. Fortunately there is no 100% intovert or he would perhaps be a severe psychotic, nor a 100% extrovert. Although many modern men, business men, &c., get dangerously near to being 100% extroverts, but happily they cannot reach it and then some crisis comes in beneficialy to pull them back.
Therefore, introversion, extroversion, and ambiversion are just temporary conditions, or I might say percentages of the flow of energy.
Also, biologically, while we are asleep, we are biologically introverted but many activities go on and when we are very active physically, emotionally, in gymnastics or sports, we are physically extroverted. So at all levels it is a momentary direction of the greatest percentage of vital, psychological, and spiritual energy. So no labels, but just an existential condition which changes anyhow, and which we can take in hand and direct. Of course, at best, someone who has an integrated personality has the power to be ambivert at will. They can be one of the two at will. (Talks on the Self)