One of Jung's most important discoveries was his realisation that by understanding the way we typically process information, we can gain insights into why we act and feel the way we do. In particular, he noted that in order to better understand ourselves we need to understand the way we characteristically perceive, and then act upon, information. Jung identified two core psychological processes that he termed:
Perceiving, which involves receiving, or taking in, information, and
Judging, which involves processing that information (e.g. organising the information and coming to conclusions from it)
Jung further identified two alternative ways of perceiving information, which he termed Sensing and Intuiting, and two alternative ways of judging information, which he termed Thinking and Feeling. Moreover, he noted that these four mental process can be directed either at the external world of people and things, or at the internal world of subjective experience. He termed this attitude towards the outer world Extraversion, and this attitude towards the inner world Introversion. Thus Jung realised the existence of these four basic psychological processes, which can be used either in the external or internal world, mean that people can use their mind in one of eight ways
Jung further noted that just as people have a preference for the hand they choose to write with, and for the foot they choose to kick a ball with, so too do people have a preference for the mental processes they use to perceive, and judge, the world. In particular, he described how the preferred use of these mental processes leads to important personality differences between people. This is the essence of Jung's theory of psychological types, which describes how our preferred mental processes for judging and perceiving the world, influence the way we typically feel, think and act in our daily lives.
After publishing his seminal work on psychological types in 1921, Jung did little further work in this area. However the importance of his work was recognised by the mother and daughter team, Elizabeth Myers and Catherine Briggs. Realising that if Jung's ideas were to have any practical application it would be necessary to develop an easy method for assessing a person's psychological type, Myers and Briggs published the first type indicator in 1949. Moreover, they developed the now famous four-letter type code as a short hand way of describing a person's type.In addition to providing a short-cut route for describing a person's psychological type, the four-letter code also reveals a set of complex personality dynamics which provide insights into the order in which the preferred psychological processes are likely to manifest themselves in the person's daily life.
The way to discover the dynamic relationship between these psychological processes is by examining the middle two preferences; SN for perceiving, and TF for judging, types. These are referred to as functions. For any type, one of these functions is dominant and this dominant function is used in the preferred world (external or internal as indicated by the EI preference), with Extraverts using their dominant function in the external world and Introverts using their dominant function in the internal world. Moreover, the function which the type code indicates is not preferred is called the auxiliary function. Thus, if either the S or N function were indicated as being the preferred function, then either the T or F function would be the auxiliary function.
So, introverts are more likely to exhibit their auxiliary function when relating to the outer world, whereas their dominant function will be used mainly when relating to the inner world, and will thus not be readily accessible for others to see. In contrast, because Extraverts use their dominant function in the outer world, it will be available for all to see.
