Function strength is a positional dichotomy that divides functions into the categories of strong and weak. Whether a function is strong or weak depends on the position it takes in your stack. Strong functions are your dominant, auxiliary, ignoring, and demonstrative functions. Weak functions are your tertiary, inferior, vulnerable, and role functions.
Note that all members of the same club (NFs, NTs, SFs, and STs) share the same strong and weak functions. Please check the additional information at the end of the page for the strong and weak functions of each type.
The strong functions are those in the dominant, auxiliary, ignoring, and demonstrative positions. Strong functions have the following qualities:
Confidence. The individual is confident in their ability to use these functions. Situations which require the use of these functions do not create anxiety or insecurity.
Thoroughness. The individual is aware of complexity and nuance in these functions. Depending on cognitive style, this nuance may be more developed along particular axes. Some of the ways in which the individual's strong functions can be more thorough include:
Being more aware of the causes and effects of information and activity involving this function. For example, ESFPs with strong Se are more aware of the effects of willpower (direct/forceful approaches to problems) than ENTPs, who use the same cognitive style but have weak Se. ENTPs, on the other hand, have strong Ne, and thus are more aware of the benefits and opportunities created by lateral thinking (indirect approaches to problems) than ESFPs, who have weak Ne.
Being more aware of contextual information involving these functions - approaching situations or handling information differently depending on the context in which it applies.
Being more aware of all of the facets of a particular idea or piece of information - for instance, perceiving multiple possible implications from a single idea or event, or seeing ways in which the same object can be used in different applications.
Being more competent in mentally testing or evaluating information, such that its truth value or usefulness can be continually re-evaluated and adjusted based on new circumstances or incoming information.
Plasticity. Specifically, I'm referring to the ability to change the use or approach of a function, grow it (make it stronger), and incorporate new information. This is distinct from re-evaluating or altering existing information, as mentioned in the last part of the 'thoroughness' bullet above. Strong functions are either 4d or 3d functions. You can read more about that in my future article about dimensionality, but for a brief overview:
4d functions are your dominant and demonstrative functions. You can think of them as your strongest functions. These functions work independently of space and time. They are just as capable of perceiving solutions for or consequences of hypothetical situations as real ones. They are also capable of being used on the spot, of learning from others, or of learning from experience.
3d functions are your auxiliary and ignoring functions. They are still strong, but not as strong as 4d functions. They lack the capacity to work in the hypothetical realm. They are primarily used for understanding and judging information that is immediately relevant to the individual, such as a situation they presently find themselves in or a conundrum they are currently working on solving. In the absence of an immediate need, they are also capable of learning from others and learning from experience.
The weak functions are those in the tertiary, inferior, vulnerable, and role positions. Weak functions, in contrast to strong functions, exhibit:
Insecurity. The individual is hesitant or embarrassed about attempting to use these functions. They prefer for information, judgments, or actions related to these functions to be provided to them from external sources. For example, NJs prefer for others to supply Se, such as giving them a motivation or reward to do something difficult, rather than having to motivate or encourage themselves. TPs prefer others to supply Fe - to keep up a positive atmosphere, for instance - rather than having to check on others' moods themselves. Note that the tertiary function is often overconfident in its abilities, so although we prefer others to "supply" this function for us, we are often not consciously aware of this desire and will often mistakenly self-report a greater deal of confidence than we actually possess when the time comes to use it.
Hollowness. Judgments and interpretations using weak functions are often black-or-white, one-sided, or incomplete. They are liable to miss (or misconstrue) important information or events. This can often lead our strong functions to make errors that they wouldn't make if they were provided with correct information. For instance, an ENFP may misapply their energy in attempting to find solutions (using strong Ne) to a problem they don't completely understand (due to weak Te). If the Te is externally sourced (e.g., an ESTJ boss explains the problem to them), their Ne is likely to be much more focused and useful.
Fatigue. All weak functions induce fatigue, exhaustion, and frustration in the individual if they are required or requested to be used longer than the individual is capable of. The tertiary function is the 'least weak', followed by the role function, then the inferior, and finally the vulnerable function, which can fatigue the individual if they even think about using it. Note that the ignoring function, although a strong function, can also induce fatigue in the individual, although this is due more to boredom and frustration rather than weakness per se. Imagine the fatigue induced by working out a weak muscle (weak functions) vs. the fatigue induced by doing something easy but tedious and perceived as useless, such as blinking rapidly for five minutes straight (ignoring function).
Rigidity. Weak functions are limited in the ways that they can grow and incorporate new information. That said, it is important to note that they do learn and grow. By focusing one's effort on the appropriate methods - that is, methods related to the ways these functions actually grow - an individual can avoid excessive fatigue and frustration and produce meaningful results. Weak functions are either 2d or 1d functions.
2d functions are your tertiary and role functions. These functions prefer to learn through direct instruction and advice. They particularly benefit from research and formal or informal education. In the absence of instruction, they are also capable of learning through direct experience.
1d functions are your inferior and vulnerable functions. They cannot grow or gain new information in any way except through direct experience. In instructing someone's 1d function, for instance, explaining something to them is useless unless you can put them in a situation that they must use the 1d function to solve. For instance, you cannot teach an INFP to build a desk without putting desk parts on the ground and making them follow the instructions step-by-step, or waiting until they figure it out through trial and error. (Of course, the INFP can also put themselves in this situation.) Once they've done it, however, they will be capable of doing it and even teaching it to others in the future.
Strong and Weak Functions by Type
NFs: ENFJ, ENFP, INFJ, INFP
Strong: Ne, Ni, Fe, Fi
Weak: Se, Si, Te, Ti
NTs: ENTP, INTP, ENTJ, INTJ
Strong: Ne, Ni, Te, Ti
Weak: Se, Si, Fe, Fi
SFs: ESFJ, ESFP, ISFJ, ISFP
Strong: Se, Si, Fe, Fi
Weak: Ne, Ni, Te, Ti
STs: ESTP, ISTP, ESTJ, ISTJ
Strong: Se, Si, Te, Ti
Weak: Ne, Ni, Fe, Fi