After Jung realized the dichotomies of Introverted and Extraverted were not enough to explain all differences in Types of people, he eventually realized that all people basically have two mutually exclusive modes of operation, or mental functions : Perception of information and Judgment of information.
Perception
Perception is the psyche's portal for perceiving information in the external and internal worlds. Perceiving is 'the process of becoming aware of things, people, occurrences [events], [meanings,] and ideas' [Isabel Myers, 1980]. In other words, perceiving is the cognitive process of gathering information and bringing it into our consciousness. Perceiving can be thought of as the modem and the monitor of the mind. Perception is called an irrational process by Jung because it is not controlled, it merely is.
Judgment
Judgment is the psyche's answer to using the information gathered by Perception. Judgment provides structure and valuation to information. Myers describes judging as "the process of coming to conclusions about what is perceived" Judgment can be thought of as the processor and programming of a computer. Judgment is called a rational process by Jung because it is controlled
Another dichotomy squaring
These two functions operate in a bipolar model - one is either Perceiving or Judging, but never both simultaneously, just like one can either be Introverting or Extraverting, but never both - you're either being loud or quiet.
If we square the P/J dichotomy to the I/E dichotomy, we end up with another four-point mandala like the subjective/objective and personal/collective mandala.
In the 8 point mandala of these two poles, we end up with four modes of operation - Introverted/Extraverted Perception and Introverted/Extraverted Judgment. We can abbreviate these four points as Pi, Pe, Ji, Je.
I'll be re-editing this post and adding more to the descriptions of P and J and Pe/Pi/Je/Ji later, so check for updates.
