According to Jung, the Ego - the "I" or self-conscious faculty - has four inseperable functions, four different fundamental ways of perceiving and interpreting reality, and two ways of responding to it.
Jung divided people into Thinking, Feeling, Sensation, and Intuition types, arranging these four in a compass.
Feeling ------|------ Thinking
The four ways of interpreting reality are the four ego-functions - Sensation, Thinking, Feeling, and Intuition. These consist of two diametrically-opposed pairs. Thinking is the opposite of Feeling, and Sensation the opposite of Intuition. So, suggests Jung, if a person has the Thinking function (an analytical, "head"-type way of looking at the world) highly developed, the Feeling function (the empathetic, value-based "heart"-type way of looking at things) will be correspondingly underveloped, and in fact suppressed. The same goes for Sensation and Intuition. Sensation is orientation "outward" to physical reality, and Intuition "inward" to psychic reality.
Jung perceived of these four Ego-functions as making up a kind of fixed dial. The upper part of the dial is shown light, meaning that it is the developed conscious faculty, and the other part dark, meaning that it is the undeveloped or suppressed unconscious faculty. (Indeed, much of Jung's work involved recognition of the dichotomy of Light and Dark, Conscious and Unconscious). The faculty which is most Conscious (in this case "Thinking") is the dominant one, or Principal function, and the other one ("Intuition") is the secondary faculty, or Auxiliary function. So we have one function in full consciousness and fully developed, another function as secondary to this, a third function, the opposite of the second, as slightly suppressed and unconscious, and the fourth, the opposite of the first, as totally unconscious.
Let us consider each of the Ego faculties in a little more detail. [Note: the following account of the four functions is based mainly on Calvin S. Hall and Vernon J. Nordby, A Primer of Jungian Psychology, (1973, New American Library), pp.98ff].
Basically, THINKING refers to the faculty of rational analysis; of understanding and responding to things through the intellect, the "head" so to speak. Thinking means connecting ideas in order to arrive at a general understanding. The Thinking-type often appears detached and unemotional. The Scientist and the Philosopher are examples of the "thinking type", which is found more commonly in men.
FEELING is the interpretation of things at a value- level, a "heart"-level rather than a "head"-level. Feeling evaluates, it accepts or rejects an idea on the basis of whether it is pleasant or unpleasant. According to Jung this is the emotional personality type, and occurs more frequently in women.
Thinking and Feeling are both rational, in that they both require an act of Judgment. Sensation and Intuition are both irrational, in that they involve no reason, but simply result from stimullii (whether external or internal) acting upon the individual.
SENSATION means conscious perception through the sense-organs. The Sensation personality-type relates to physical stimulii. But there is a difference according to whether the person is an introvert or an extrovert.
So we could have an Introverted-Sensation type, such as an artist, who experiences the physical world (sensation) from the perspective of the psychic or inner consciousness (introversion). As opposed to this, the Extroverted-Sensation type would be the person who is a simple materialist or hedonist, interested only in physical or pragmatic things. This type tends to be realistic and practical. At worst, one may be crudely sensual. This personality-type occurs more often in men.
Finally, INTUITION is like sensation in that it is an experience which is immediately given to con-sciousness rather than arising through mental activity (e.g. thinking or feeling). But it differs in that it has no physical cause. It constitutes an intuition or hunch, a "gut"-level feeling, or an "ESP" experience. It is the source of inspiration, creativity, novel ideas, etc. According to Jung, the Intuitive type jumps from image, is interested in a while, but soon loses interest.
With the four Ego-faculties of Thinking, Feeling, Sensation, and Intuition we have a basic classification of modes of consciousness; one that has been postulated under various forms (of which Jung's is only the most recent) for centuries.
For example, these are the four elements of the Greeks, with which the ego-functions can to some extent be indentified with. Indeed, Western occultists have given these elements psychological characteristics for some time. The four elements themselves are more representative of subjective or human psychological and physiological (the four "humors") than objective "scientific" physical factors.
Jung also speaks of Extraversion and Introversion as the two ways of responding to the world. Extraversion and Intoversion, which again could be seen as diametric opposites, the Extravert again being orientated out to the physical, the Introvert orientated in to the psychic. In this case what is being described is the direction of the consciousness. So we could have an Introverted-Sensation type, such as an artist, who experiences the physical world (sensation) from the perspective of the psychic or inner consciousness (introversion). As opposed to this, the Extroverted-Sensation type would be the person who is a simple materialist or hedonist, interested only in physical or pragmatic things.
Again, there would be the Extroverted-Intuition type, who has psychic experiences or revelations, and is able to easily convey them out to others at the social or interpersonal level. Most professional clairvoyants and psychics, and the founders and Gurus of various religious sects, would seem to fall into this category. One could call this the "prophetic personality". As opposed to this would be the Introverted-Intuition type, who is caught in the psychic experiences, and not able to share them very well with others. Many creatively original schizophrenics would belong to this group. Schizophrenic experiences, it should be pointed out, are real experiences of the psychic worlds. The term "hallucination" is meaningless to the occultist or esotericist. Because all experiences are real, there is no such thing as a "hallucination". What the materialist and the sceptic calls a hallucination is simply an experience of a reality of one of the psychic worlds, which of course being a non- physical reality, is inexplainable and threatening to the materialist, and hence dismissed as "hallucination".
My interepretation is that introversion and extraversion have nothing to do with the ego-faculty. They are actually expressions of the energy state and energy flow of the etheric bodies. When tehse principels direct the psychic consciousness out into the inter-personal material world the result is an extravertic or outward flowing personality. When they directs the experience it receives strongly back into the Psychic principles, the result is the introvertive or inward flowing personality.