All of MBTI can be summed down to this concept: that human life consists of receiving information and reacting to information, and that every individual has natural preferences in their approach to both.
Of the functions, there are two types: judging and perceiving.
Perceiving functions are used to receive information, judging functions are used to react to information.
(“Information” in this sense is not specific to data, but also includes emotions, patterns, details of reality, ideas, concepts, sensory stimulation, etc.)
The Function Stack
The order by which you prefer certain functions is what creates your personality type. This is referred to as the “function stack” where each type has an order of ranking the cognitive functions.
The lower the ranking of a particular function, the less comfortable or natural the use of that function is to the individual.
For example, The INTP function stack leads with Ti (introverted thinking) as the primary function, Ne (extraverted intuition) as auxiliary, Si (introverted sensing) as tertiary, and Fe (extraverted feeling) as inferior.
But what are the functions exactly?
Introverted Thinking (Ti, Judging Function)
Primary: INTP, ISTP
Auxiliary: ENTP, ESTP
Tertiary: INFJ, ISFJ
Inferior: ENFJ, ESFJ
Introverted thinking is the cognitive function which seeks information and logic for personal use.
It values knowledge for the sake of knowledge, and it seeks to define.
Both INTPs and ISTPs are naturally great at understanding underlying logical principles and are very likely to spot any inconsistencies in arguments.
“...if their criteria, categories, and logic are hidden from view and seem to serve the goal of creating a personal understanding through subjective [mental], internally consistent, and precise logic - in the interest of discovering truth by naming, defining, and understanding all of the pieces of the puzzle - then it’s pretty clear that it is well-developed introverted thinking that we’re witnessing.” - Mark Hunziker
Extraverted Thinking (Te, Judging Function)
Primary: ENTJ, ESTJ
Auxiliary: INTJ, ISTJ
Tertiary: ENFP, ESFP
Inferior: INFP, ISFP
Extraverted thinking is the cognitive function that seeks to apply logical analysis and principles to external systems and processes.
Personality types that have primary Te - ENTJ and ESTJ - are naturally talented at creating and organizing information into a practical and logical framework.
“When we observe someone who tends to organize the world around them, creating systems that support people and processes, based on shared rules, standards, structures, and procedures - someone for whom an orderly environment seems paramount and who seeks to create that order through regulation, planning, and enforcements - we should suspect that their extraverted use of thinking is highly developed.” - Mark Hunziker
Introverted Feeling (Fi, Judging Function)
Primary: INFP, ISFP
Auxiliary: ENFP, ESFP
Tertiary: INTJ, ISTJ
Inferior: ENTJ, ESTJ
Introverted feeling seeks personal harmony. Meaning, the introverted feeling function deals with assessing core personal values.
Personalities with a primary Fi function (ISFP and INFP) might be mistaken as self-centered because of their more private nature.
However, this is only because they are less likely to be persuaded by the emotional state of their environment and are more naturally concerned with internal balance.
“We could expect, then, that they would not tend to try to win others over to their point of view. They’d often go along with what they assume to be others’ deeply held convictions - as long as their own are not violated. And when they do lead, it would tend to be quietly and subtly by example or through body language. Since they are sustained by their inner harmony, they might strike us as serene, and it would not be surprising to find that they connect with others mostly in one-on-one situations based on high levels of personal trust and authenticity.” - Mark Hunziker
Extraverted Feeling (Fe, Judging Function)
Primary: ENFJ, ESFJ
Auxiliary: INFJ, ISFJ
Tertiary: ENTP, ESTP
Inferior: INTP, ISTP
Extraverted Feeling is the function-attitude which is primarily focused on creating harmony within interpersonal relationships.
Those who have a natural, higher preference for Fe are typically those who are more aware of the emotions of others and the overall emotional atmosphere of their environment.
The two personalities that lead with Fe are ESFJ and ENFJ. Both of these personalities have a natural tendency to be welcoming and warm as well as have the priority of providing others with opportunities to express their emotions.
“They would probably make for gracious and conscientious hosts, excluding warmth as they actively seek to connect with other people. Their concerns for others’ needs would probably be obvious, and they would make those around them feel accepted and validated.” - Mark Hunziker
Introverted Sensing (Si, Perceiving Function)
Primary: ISFJ, ISTJ
Auxiliary: ESFJ, ESTJ
Tertiary: INTP, INFP
Inferior: ENFP, ENTP
Introverted sensing is the function-attitude that is naturally aware of the order of events and the details of past experiences.
Individuals that prefer Si over other functions can easily recall the motions, scents, sounds, and images they experienced. When telling of an event, they will often make you feel as if you had experienced it yourself - even if you hadn’t.
ISFJ and ISTJ both lead with Si as their primary function. They tend to love routine and have very organized minds.
“And this would likely make them very stable and reliable, most comfortable in activities they’ve encountered before, perhaps a bit slow to embrace new things…” - Mark Hunziker
Extraverted Sensing (Se, Perceiving Function)
Primary: ESFP, ESTP
Auxiliary: ISFP, ISTP
Tertiary: ENFJ, ENTJ
Inferior: INFJ, INTJ
Extraverted sensing is the function-attitude which is primarily concerned with seeking external experiences and stimulation.
Individuals that prefer Se over other functions are more naturally aware of their environment, the details of their surroundings, and are very practical and matter-of-fact.
ESFPs and ESTPs are very much grounded in reality, however, this does not mean they don’t also love to rationalize or be imaginative. It simply means they prefer to do so when it is based on the reality of the world around them.
“We can reasonably expect that people with highly-developed extraverted sensing would tend to be engaged in ‘smelling the roses’ in the moment, rather than spending much time reminiscing, fantasizing, or planning…” - Mark Hunziker
Introverted Intuition (Ni, Perceiving Function)
Primary: INFJ, INTJ
Auxiliary: ENFJ, ENTJ
Tertiary: ISFP, ISTP
Inferior: ESFP, ESTP
Introverted intuition is the function-attitude which is focused on acquiring and creating concepts, theories, and hypothetical scenarios. It is extremely imaginative, based less on what is and more of what could be. In conversation, it is often driving things forward - constantly searching for the deeper meaning.
“...if they seem to consider everything as being relevant, even unconscious material and the distant past and future - all in the interest of imagining distant, vague possibilities, gaining a personal sense of knowing, and divining meaning and purpose - this evinces use of well-developed introverted intuition.” - Mark Hunziker
Extraverted Intuition (Ne, Perceiving Function)
Primary: ENFP, ENTP
Auxiliary: INTP, INFP
Tertiary: ESFJ, ESTJ
Inferior: ISFJ, ISTJ
Extraverted intuition seeks to understand patterns, possibilities, and interconnections based on the external world.
Unlike Se, Ne is more aware of the potential of objects rather than the details of its current existence, and focuses more on the underlying principles and patterns of the things which it is giving attention to.
ENTP and ENTP both have Ne as their primary function and both of these types are naturally great at considering possible alternatives to given situations. They frequently are creating lots of theories and ideas to be enacted in the real world.
“So when we observe someone whose interest is in generating practical, real-world, tangible, ‘doable’, demonstrable, and explainable options - envisioning all scenarios, entertaining any possibility, and enabling others to see and experiment with an array of endeavors - we can deduce that their extraverted intuition is well-developed and active.” - Mark Hunziker
To truly get a solid understanding of the functions, one must do so in practice, through self-reflection and observing others. Some functions (especially intuition) are very difficult to articulate into detail and are much easier to understand by practice.
I highly suggest you keep your type in mind while doing so, especially since you will use certain functions as you go about your observations. (Ne users will see patterns and Se users will pay attention to details, etc.)
As you may already be aware, the psyche can be divided into two parts: ego and shadow. This is also something you may want to keep in mind.
While each person has four cognitive functions that are naturally more preferred (ego), the four other functions lie within your shadow - the unconscious part of the psyche.
Taking the INTP again for example:
As you can see, the shadow functions are reflective in dichotomy, but opposite in whether they are introverted or extroverted. Thus, development of the shadow functions would require a great amount of uncomfortable situations for an individual. But with time, the development of these functions can and does tend to happen.
Know that I am not a professional typologist, but through years of research I will say that understanding the functions (even on a basic level) will help you better understand both yourself and the world around you.