Functions
Carl Jung defined the four cardinal functions of all people -- thinking, feeling, sensing and intuiting. We all have each of these, and they are present. An important aspect of this layer is the notion of the "inferior function". Generally, it lies on the opposite side of a person's strongest function. Thus, a person whose strongest function was thinking may very well have a corresponding weakening of their feeling function.
Feeling
This is feeling in the sense of emotion, not in the sense of touch. The capacity to tolerate and express emotions resides in this quadrant.
Sensing
This is physical sensation, being in the body and attending to the messages from our body.
Thinking
The intellectual faculty, thinking, words. Building the mental map. This is usually strong, and sometimes exaggeratedly so, in modern Western culture.Intuiting
Intuition, spontaneity, creativity, the unknown. This faculty is often neglected, and even atrophied, in our culture.

