The MBTI system can be an incredibly valuable tool to help you understand yourself and the people around you. But just like with any tool, its value depends on the skill of the person wielding it. So learn here how MBTI works under the hood.

As you can read in my previous article, MBTI or Myers-Briggs Type Indicator is a system that categorizes people in 16 different personality types, according to how they generally perceive the world and make decisions. The MBTI system is based on the cognitive functions as identified by Swiss psychiatrist and psychoanalyst Carl Jung.

The cognitive functions

Jung identified different cognitive functions people use to (1) perceive the world and (2) make decisions. These are referred to as observer functions and decider functions, respectively.

Each of these functions can be mostly extroverted or mostly introverted. In this context, extroverted means directed at the outside world and introverted means directed at the self.

Before we go on, let me stress that everybody has everything and everybody can do everything, but our propensities are not all the same. And that's what this is all about.

Observer functions

The first distinction that is being made is between intuition (N) and sensing (S). If your propensity is to perceive the world mostly in the abstract, paying attention to patterns, forming theories of understanding, ideas, and visions, you are an intuitive.

If you perceive the world mostly as concrete reality, paying attention to how things are, what they look like, smell like, feel like and how things fit together, you are a sensor.

If your observer function is extroverted, you lean towards gathering from the outside world, whereas if your observer function is introverted, you are more concerned with organizing what you have already gathered.

So introverted intuition (Ni) focuses on organizing ideas and patterns into one coherent worldview, whereas extroverted intuition (Ne) leans more towards exploring new ideas.

Similarly, introverted sensing (Si) focuses on organizing what has already happened in the form of facts, data and other information, especially about past events and circumstances. Extroverted sensing (Se) leans more towards exploring new experiences, actions and contributions in the physical world.

Decider functions

Now that we've looked at the different tendencies to perceive the world, let's look at the different modes of making decisions. These cognitive functions are named thinking (T) and feeling (F). Of course, being a feeler doesn't mean you don't think, and being a thinker doesn't mean you don't feel. These are just (poor) names for the different propensities.

If you tend to make your decisions using logical reasoning, you are concerned with 'what works' and some people would describe you as a ruthless rationalist, you are a thinker in MBTI. If you tend to let harmony be the deciding factor when you make decisions, you are considered a feeler.

If your decider function is extroverted, you prioritize the outside world in your decision making, whereas if your decider function is introverted, you are more concerned with the self.

In other words, extroverted thinking (Te) is decision making based on what the user thinks will work for the tribe, whereas introverted thinking (Ti) reasons more from the self.

Similarly, extroverted feeling (Fe) is decision making based on tribal harmony, whereas for introverted feeling (Fi) the person's own value system is prioritized over the tribe's.

Function stacks and the letters

If this all makes sense, it's time to discuss how it all works together in the function stack. The function stack is the arrangement of cognitive functions that make up the personality in the MBTI system.

So, how does this work? Let's run through it taking the example of the ISTJ.

Everybody has one function that is generally dominant at the top of the function stack. The dominant function determines whether the personality is generally introverted or extroverted.

For the ISTJ, the dominant function is the observer function introverted sensing (Si), which means this personality as a whole is introverted.

If the dominant function is a decider, the second main function must be an observer function and vice versa.

For the ISTJ, since his dominant function is the observer function Si, his second function must be the decider T.

Everyone has some way they predominantly use to engage the outside world (i.e. extraversion), whether it is their dominant function or not. So either the first or the second function must be extraverted.

Seeing how the ISTJ's dominant observer function is introverted, his extroverted function must be his main decider function: extroverted thinking (Te).

And that is basically it.

The other two functions follow from what we already have.The third function mirrors the second function and the fourth function mirrors the first function.

After all, any dominant function is overdeveloped by definition, which means that the polar opposite of that function is underdeveloped by definition.

The ISTJ's secondary function is extroverted thinking (Te), which means the opposite decider function, introverted feeling (Fi) must be his third function. And seeing how his dominant function is introverted sensing (Si), his weakest function must be the polar opposite observer function: extroverted intuition (Ne).

Know thyself

I understand that this may all be a bit complex, especially if this is the first time you're hearing about this. But trust me when I say that it will all start making so much more sense when you've successfully managed to type yourself and a few other people you know well.

You will start to see why people do what they do, what they're uniquely good at, what they find important, what appeals to them and how they ultimately make decisions. And you will also learn to see your own strengths and blind spots, which is incredibly valuable if you're looking to improve yourself.

That is why in my next article, I'm going to give you a step by step approach that teaches you how to type people, so you won't have to depend on online tests anymore.