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.